It does look like there have been changes to the way Java handles dates in recent versions which has had knock-on effects on Lucee.
https://luceeserver.atlassian.net/browse/LDEV-4658
FWIW, Lucee 6.2 on Java 21 still returns false for:
Dump( IsDate( "Jan 4, 2018 12:00 AM" ) )
… whereas Lucee 5.4.6.9/Java11 returns true.
But if you remove the comma, 6.2/Java21 reports true:
Dump( IsDate( "Jan 4 2018 12:00 AM" ) )