quick summary
After finally transferring my Survival world to Bedrock after finishing my 5×5 map wall, I immediately noticed that all but four of my maps failed to transfer at all, both the item frames maps and stored maps (see the pictures for more information).

more information
One thing I know for sure is that the maps that successfully converted were created BEFORE tu54 (the update that added explorer maps) and the maps created on or after tu54 completely deleted themselves.

steps to reproduce
This will be tricky to do unless you have an world made before TU54 with at least one map already created
1) Load a pre-TU54 world with at least one preexisting map
2) Create a new map (in a new chunk, making it in the same chunk will copy the map)
3) put both maps in item frames
4) convert the world
5) notice how the new map doesn't exist