Monuments of zero size are reported repeatedly on reddit.

[AbcLmn18]

Example.

These posts show up roughly every week and I was quite certain that there's a mod that's causing them but this time there's low-quality evidence that it's vanilla. Which makes sense given the sheer amount of posts.

I don't have any further concrete evidence but I thought it's a good idea to attract attention to this issue as it seems popular. Please close if it's already known and/or known to be a mod issue (in that case, people would probably love to know what mod this is!).