Basically, update the mod to 1.19.4 and, preferentially, 1.20. This mod improves how maps work in the Nether and in other cave dimensions by showing only the blocks below a certain height, which can either be the height the map was created at or a value specified in the config.
The author are willing to accept PRs for update (attached picture)
MR: https://modrinth.com/mod/better-nether-map
GH: https://github.com/Jummit/better-nether-map