ADDITIONAL SYSTEM INFORMATION :
Reproduced on Windows 10, but I do not expect the OS to be relevant.

Reproduced with Java 1.8.0_222 and "17-ea" 2021-09-14:

>java -version
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_222-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.222-b10, mixed mode)

>java -version
openjdk version "17-ea" 2021-09-14
OpenJDK Runtime Environment (build 17-ea+5-266)
OpenJDK 64-Bit Server VM (build 17-ea+5-266, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
When the following conditions meet:

* Use a `%a` conversion with the `0` flag and a width (e.g., `"%012a"`)
* When the result has a sign character (`-`, or `+` or ` ` (space) with the respective flags)

then 1 too many padding '0's is inserted, resulting in a formatted string 1 char longer than the specified width.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Using the provided test file `test/Test.java` below:

$ javac -d bin test/Test.java
$ java -cp bin test.Test

Observe that the right-most `|` do not all align, even though they should (in each block). Every line marked with a `// !!` comment in the source code emits one character '0' too many in the padding.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
|0x000000000000000000000000.0p0|
| 0x00000000000000000000000.0p0|
|+0x00000000000000000000000.0p0|
|-0x00000000000000000000000.0p0|
|-0x00000000000000000000000.0p0|
|-0x00000000000000000000000.0p0|
|-0x1.0p1 |
| 0x1.0p1 |
|0x000000000000000000000001.0p1|
|-0x00000000000000000000001.0p1|
| 0x00000000000000000000001.0p1|
|+0x00000000000000000000001.0p1|
|-0x1.26580b480ca46p30 |
| 0x1.26580b480ca46p30 |
|0x00000000001.26580b480ca46p30|
|-0x0000000001.26580b480ca46p30|
| 0x0000000001.26580b480ca46p30|
|+0x0000000001.26580b480ca46p30|

|0x000000.0p0|
| 0x00000.0p0|
|+0x00000.0p0|
|-0x00000.0p0|
|-0x00000.0p0|
|-0x00000.0p0|
|-0x1.0p1 |
| 0x1.0p1 |
|0x000001.0p1|
|-0x00001.0p1|
| 0x00001.0p1|
|+0x00001.0p1|
|-0x1.2p30 |
| 0x1.2p30 |
|0x00001.2p30|
|-0x0001.2p30|
| 0x0001.2p30|
|+0x0001.2p30|

ACTUAL -
|0x000000000000000000000000.0p0|
| 0x000000000000000000000000.0p0|
|+0x000000000000000000000000.0p0|
|-0x000000000000000000000000.0p0|
|-0x000000000000000000000000.0p0|
|-0x000000000000000000000000.0p0|
|-0x1.0p1 |
| 0x1.0p1 |
|0x000000000000000000000001.0p1|
|-0x000000000000000000000001.0p1|
| 0x000000000000000000000001.0p1|
|+0x000000000000000000000001.0p1|
|-0x1.26580b480ca46p30 |
| 0x1.26580b480ca46p30 |
|0x00000000001.26580b480ca46p30|
|-0x00000000001.26580b480ca46p30|
| 0x00000000001.26580b480ca46p30|
|+0x00000000001.26580b480ca46p30|

|0x000000.0p0|
| 0x000000.0p0|
|+0x000000.0p0|
|-0x000000.0p0|
|-0x000000.0p0|
|-0x000000.0p0|
|-0x1.0p1 |
| 0x1.0p1 |
|0x000001.0p1|
|-0x000001.0p1|
| 0x000001.0p1|
|+0x000001.0p1|
|-0x1.2p30 |
| 0x1.2p30 |
|0x00001.2p30|
|-0x00001.2p30|
| 0x00001.2p30|
|+0x00001.2p30|


---------- BEGIN SOURCE ----------
package test;

public class Test {
  public static void main(String[] args) {
    System.out.println(String.format("|%030a|", 0.0));
    System.out.println(String.format("|% 030a|", 0.0)); // !!
    System.out.println(String.format("|%+030a|", 0.0)); // !!
    System.out.println(String.format("|%030a|", -0.0)); // !!
    System.out.println(String.format("|% 030a|", -0.0)); // !!
    System.out.println(String.format("|%+030a|", -0.0)); // !!

    System.out.println(String.format("|%- 30a|", -2.0));
    System.out.println(String.format("|%- 30a|", 2.0));
    System.out.println(String.format("|%030a|", 2.0));
    System.out.println(String.format("|%030a|", -2.0)); // !!
    System.out.println(String.format("|% 030a|", 2.0)); // !!
    System.out.println(String.format("|%+030a|", 2.0)); // !!

    System.out.println(String.format("|%- 30a|", -1234567890.012345678));
    System.out.println(String.format("|%- 30a|", 1234567890.012345678));
    System.out.println(String.format("|%030a|", 1234567890.012345678));
    System.out.println(String.format("|%030a|", -1234567890.012345678)); // !!
    System.out.println(String.format("|% 030a|", 1234567890.012345678)); // !!
    System.out.println(String.format("|%+030a|", 1234567890.012345678)); // !!
    
    System.out.println("");

    System.out.println(String.format("|%012.0a|", 0.0));
    System.out.println(String.format("|% 012.0a|", 0.0)); // !!
    System.out.println(String.format("|%+012.0a|", 0.0)); // !!
    System.out.println(String.format("|%012.0a|", -0.0)); // !!
    System.out.println(String.format("|% 012.0a|", -0.0)); // !!
    System.out.println(String.format("|%+012.0a|", -0.0)); // !!

    System.out.println(String.format("|%- 12.0a|", -2.0));
    System.out.println(String.format("|%- 12.0a|", 2.0));
    System.out.println(String.format("|%012.0a|", 2.0));
    System.out.println(String.format("|%012.0a|", -2.0)); // !!
    System.out.println(String.format("|% 012.0a|", 2.0)); // !!
    System.out.println(String.format("|%+012.0a|", 2.0)); // !!

    System.out.println(String.format("|%- 12.0a|", -1234567890.012345678));
    System.out.println(String.format("|%- 12.0a|", 1234567890.012345678));
    System.out.println(String.format("|%012.0a|", 1234567890.012345678));
    System.out.println(String.format("|%012.0a|", -1234567890.012345678)); // !!
    System.out.println(String.format("|% 012.0a|", 1234567890.012345678)); // !!
    System.out.println(String.format("|%+012.0a|", 1234567890.012345678)); // !!
  }
}


---------- END SOURCE ----------

FREQUENCY : always