RE: Activity Timeline Notes Preview
We have the same issue here. We are running version 2020 release wave 1 enabled, Server version: 9.1.0000.19857
I noticed that in our organization it did occur on Account and Contact but not on the Lead entity.
I looked at the differences between the forms and noticed that the widget 'Activities' > properties > tab activities > on Lead had 'Display activities using default fields' set whereas it was set to 'Display activities using Card Form' on Account and Contact.
When I checked the Card Form for Phone Calls it had the following set
Row 1: Call From - Actual End
Row 2: Call To
Row 3: Subject
Row 4: Description
Now when I flipped Row 3 and 4 the Description was visible again. It appears that the available space to display went from 4 rows to 3... at least this did fix it for us.