I have spent about 2 days on this problem now after WordPress updated to 5.7 on April 20, 2021. It was working perfectly before. It is still not working after everything recommended here. I also purchased an SSL certificate around the same time, so for a while I was thinking that it’s some sort of a conflict with that. I HAVE been able to narrow down that it’s the TYPE of link it doesn’t like and it is the Custom HTML block that is causing the issue in block editor. When I tried putting a simple piece of html in there it worked fine, but it is blocking my affiliate links from Amazon. One is a div, and one is is iframe. However, I can use a standard http link and link it to Amazon without problems.
This problem does not appear in classic editor at all which makes me think the issue is with the block editor and not with the link.
So for now, my workaround is using classic editor and adding the links directly to the code. I cannot go back to block editor after to edit because it will destroy the links in the document and fail to save and publish. What a frustrating experience and a huge waste of time, especially when everything was working fine prior to this update.