This look like a lots like this #73 Did you do the same manipulation with minecraft map ?
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TheNety commented
Can confirm. It sometimes creates the deformatted map right after generating it. Without need of placing it and destroying like in #73.
Vlammar commented
Does this happen all the time ? I tried to replicate this bug but i didn't manage to. Can you put the full command line you used (link included if possible) ? Just to make sure you are on 1.15.2 on spigot or paper? and you are using IoM 4.0pre2?