With the implementation of ID 2020 my team and I have run into a problem saving .icma files. (We are currently using PC's and I have not experimented with a mac yet.) I have found 2 things that happen based on what you do prior to changing the assignment location.
We have opened our files (all ID 2019 versions), all of which already have stories and assignments attached to them. After making our changes, and before saving the file I go to "Change Assignment location" and the file name is extremely long as it includes the complete path, along with the name of the file.
The second issue is changing the location after you save the file. Clicking on the "change file location" will only show a portion of the file name with a period at the end but not include the file type (icma). I did do a save without adding the file extension but windows shows it as an unknown file type.