I assume from your description this works correctly in 15.1 and has the issue in 19.4 thus this is going to be a software bug. These are known as PR (problem reports) in Junos. I don't see your particular one listed in my searches of the public PR database located here.
https://prsearch.juniper.net/InfoCenter/index?page=prsearch
You can report this bug via a JTAC ticket and they can confirm if it is already known or create a new PR if needed for the software team to fix the issue.
https://my.juniper.net/#dashboard/servicerequests