This is not a simple question. But let me share the following pointers.
Business owners complain that they are not able to get unwanted (read: negative) reviews removed from their profiles.
Google Maps says they will only take down reviews that violate the Guidelines issues by Google.
An AI system is monitoring all contributions and the system do make mistakes.
I have helped numourous Local Guides with hidden reviews. And in (I guess) 90% of the cases the Local Guided was of the opinion that they know all the guidelines and that none of their reviews violated any guidelines. After close inspection many review guidelines were actually blatantly violated.
The guidelines are complex and not very well documented and explained. So it is not surprising that guidelines get violated.
Also, keep in mind that the trigger might not be in the review text itself. Try deleting the attached photos and check if your review goes public. If so, try adding the photos back one by one and keep checking. This way you can figure out which photo was the culprit.
Furthermore, the spam filter looks at your overall performance and track record as a local guide. So completely unrelated violations can also cause reviews to be removed from public view or particular or all future reviews from the LG get blocked.
I hope this helps a bit even though I’m unable to give you a definitive answer.
Cheers
Morten