Triggering Sidewarps between different (or even the same) dmap causes the custom minimap to vanish during screen transition. In older versions of ZC this did not happen.
This was actually part of a fix for an issue that I reported a while ago.
The problem was that if you side-warped to a different level which also used a different minimap, the different minimap would show during the warp transition, and then disappear afterward. What that meant was that if you hadn't already picked up the map item for that different level, you'd be given an unintended preview of that level's layout during the warp.
Having the minimap disappear was "not the ideal solution," at least according to the developer, but apparently the only practical one given the state of the code they were working with.
Maybe it could be revisited though, and kept on display if the DMap level numbers match? I really have no idea. The only thing I know about the ZC source is that, if it should be an easy fix, it's usually an unprecedented undertaking.