Fix for this issue is now available in the latest update of InDesign 2021 – that is, InDesign 16.2.1
In the Creative Cloud Desktop application, click on the Updates section on the left. You should be able to see the Update available for InDesign. Please update your InDesign application to this version to try out the fix.
Let us know in the comments section if you still face this problem.
Adobe InDesign Team
An error occurred while saving the commentHenk commented
As an additional heads up, this also happens while working in Indesign with "high quality display" on. When exporting to 300DPI it's gone. On lower resolution the lines return.