Can you please let us know if any of the below workarounds help:
1. On refreshing the contents of the folder (where these indesign files and idml are saved), the icons of indd and idml files should appear again
2. On the 2nd launch of InDesign the folder icons should reappear.
An error occurred while saving the commentAngela Maria commented
This is happening to me as well and also my files are not opening with the updated versions.