I've been playing my old LAN world after upgrading it to 1.13 (via the optimize world function) and started adding named map markers to all locations in my huge map room (which is a really nice feature btw), but unfortunately some markers disappear after reloading the world. They can only be re-added to the map by destroying the banner, placing it again and readding the marker icon back to the map. Just right-clicking the missing marker banner does nothing. This bug seems to be specific to certain locations in the world and is repdroducible consistently at those locations just by reloading the world. I couldn't find any relationship to banners being near a chunk border, being placed on certain special coordinates or blocks, having a certain orientation or a certain name, this issue seems to be quite random. I wasn't able to reproduce it in any world created in 1.13 or its snapshots/pre-releases.

Other people appear to be having the same problem, as shown here (thx sab39): https://imgur.com/a/ugewLwj

Unfortunately can't upload the world because it is about 400 MB in size. If anyone has a smaller world where this can be reproduced, please upload or link it here.