ADDITIONAL SYSTEM INFORMATION :
openjdk version "11.0.11" 2021-04-20
OpenJDK Runtime Environment 18.9 (build 11.0.11+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.11+9, mixed mode)


OS Name: Microsoft Windows 10 Enterprise
OS Version: 10.0.19042 N/A Build 19042

System Model: HP EliteBook 850 G7 Notebook PC
System Type: x64-based PC
Processor(s): 1 Processor(s) Installed.
                           [01]: Intel64 Family 6 Model 142 Stepping 12 GenuineIntel ~1803 Mhz
BIOS Version: HP S73 Ver. 01.03.02, 11/4/2020

A DESCRIPTION OF THE PROBLEM :
If we use "Instant.now().toString()" it is differently represented:
"2021-05-04T12:52:25.707903600Z" vs "2021-05-04T12:52:25.708903Z"
Ending "00 suffix " looks s

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
It was checked in OpenJDK 11 and OpenJDK16.
Just print Instant.now().toString() in the circle of >=1000 items

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
All records should look the same with nanos or without them, but not with ending zeros:
"2021-05-04T12:52:25.707903600Z" vs "2021-05-04T12:52:25.708903Z"
ACTUAL -
...
2021-05-04T12:52:25.723904200Z
2021-05-04T12:52:25.724904Z
...
2021-05-04T12:52:25.724904Z
2021-05-04T12:52:25.725904900Z
...
209 from 1000 are truncated

---------- BEGIN SOURCE ----------
public class Main {

    public static void main(String[] args) {
        var truncatedAmount = 0;
        var testedAmount = 1_000;
        for (var i = 0; i < testedAmount; i++) {
            var instantAsString = Instant.now().toString();
            System.out.println(instantAsString);
            if (instantAsString.length() <= 27) {
                truncatedAmount++;
            }
        }
        System.out.printf("%s from %s are truncated%n", truncatedAmount, testedAmount);
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
In our case this issue causes the incorrect bahaviour for com.fasterxml.jackson.datatype.jsr310.ser.InstantSerializer.
Custom TruncatedInstantSerializer was introduced by using Instant.truncatedTo(ChronoUnit.MICROS) to make the results consistent.

FREQUENCY : always