Some footnotes disappeared without reason then appeared back when I changed something in the linked paragraph. InDesign didn't display any kind of warning about this problem.
Part of this issue was fixed with the latest release of InDesign 15.0.1
For all new files created with ID 15.0.1 the issue will not come. For older files opened in ID 15.0.1, you will have to resize, move or do some text editing operation to make the footnote appear
In case you are still facing the issue in new files created post ID 15.0.0 , please share a sample file
Gregor Fellenz commented
This bug exists in CC 2021. A sample file is listed in Bug ID - ID-4218499 (Adobe Prerelease)