i know this problem, the switch mc has a feature where it will "lock" worlds that either take too much memory to load, or you deleed the data pack that they used. this likely is a memory issue, so if you go into your settings, and clik on a small folder, you will gain access to all worlds no matter what, this will let you delete old, unused, or ram hungry worlds.
haooy gaming,
-Vanta Smithing LLC IT.