ADDITIONAL SYSTEM INFORMATION :
Windows 10, although the OS is probably irrelevant.

Reproduced both with Java 8 and 17ea:

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)

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 all the following apply:

* using the '%g' conversion with a BigDecimal,
* the value of the BigDecimal is < 10^-4 (resp. 10^precision)
* the *rounded* value to the specified precision = 10^-4 (resp. 10^precision)

Then, Formatter will format the value with scientific notation instead of fixed notation (resp. fixed instead of scientific).

Note that the JavaDoc clearly specifies that:

> After rounding for the precision, the formatting of the resulting magnitude m depends on its value.

Since the value is rounded up to 10^4 (resp. 10^precision), it should be displayed with fixed (resp. scientific) precision.

Floats and Doubles are correctly displayed depending on their value after rounding.

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

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

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
0.000100000
0.000100000
0.000100000

1.00000000e+14
1.000e+04
1.00000000e+14
ACTUAL -
0.000100000
0.000100000
1.00000e-04

1.00000000e+14
1.000e+04
1000000000

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

import java.math.BigDecimal;

public class Test {
  public static void main(String[] args) {
    // Rounding to 10^-4
    System.out.println(String.format("%g", 0.00009999999999999995));
    System.out.println(String.format("%g", 0.00009999999f));
    System.out.println(String.format("%g", new BigDecimal("0.00009999999999999999995"))); // !!
    
    System.out.println("");

    // Rounding to 10^precision
    System.out.println(String.format("%.9g", 999999.9999994322e+8));
    System.out.println(String.format("%.4g", 999.961e1f));
    System.out.println(String.format("%.9g", new BigDecimal("999999.999999432168754e+3"))); // !!
  }
}

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

FREQUENCY : always