ADDITIONAL SYSTEM INFORMATION :
The other item that has recently changed is that this code is now running on Windows 10 1809 whereas the JDK-10 run was on Windows 10 1803. (In the case the change is because of Windows and not because of JDK-11.)

A DESCRIPTION OF THE PROBLEM :
I have not changed (nor recompiled) my code (these class files were probably compiled with JDK-9), here is a date formatted from JDK-10 and before (was the same all the way back to 8) and a similar date as formatted in JDK-11. Note the extra period after Oct
JDK-10.0.2: (started 18.Sep.19-08.30.44)
JDK-11: (started 18.Oct..04-14.59.47)

Sample lines of code doing the formatting are:
private static final SimpleDateFormat dateFormat = new SimpleDateFormat("yy.MMM.dd-HH.mm.ss");
writer.write("Date: " + dateFormat.format(new Date()) );

REGRESSION : Last worked in version 10.0.2

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run program with the supplied lines of code.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
There would not be an extra period in the output.
So it might look like (started 18.Oct.04-14.59.47)
ACTUAL -
There is an extra period in the output:
(started 18.Oct..04-14.59.47)

---------- BEGIN SOURCE ----------
private static final SimpleDateFormat dateFormat = new SimpleDateFormat("yy.MMM.dd-HH.mm.ss");
writer.write("Date: " + dateFormat.format(new Date()) );
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None yet explored... unwilling to second guess the output format of a system library.

FREQUENCY : always