SpamAssassin released a new version of their spam-detection tool that implements new rules, including this "FONT_INVIS_MSGID" message.
After some research, the only description I could find is "Invisible text + suspicious message ID". SpamAssassin always has been fuzzy in the descriptions and this isn't really telling much.
The mail-tester.com integration doesn't offer a description for this message yet but they may have some more knowledge on the matter. You can ask they for a better description of the problem here: https://www.mail-tester.com/contact
We will do some tests on our end but for now I couldn't have this issue with the latest version of AcyMailing. You could duplicate your email and remove some parts until the "FONT_INVIS_MSGID" message disappears in the spam-test to find the part of the email that SpamAssassin doesn't like.