sharepoint online – tags are not supported in the HTML code specified in the Content property or the Content Link property

I have two SharePoint Online site collections. One is rather old, the other was created about two hours ago.

There is an HTML file in _catalogs / master / webparts / test.html with the following content. Note that there is no tag.

TEST

Now that I'm adding a CEWP to a page and referencing this simple HTML file, the WebPart displays an error:

< FORM > tags are not supported in the HTML code specified in the
Content property or content link property. You can remove the
< FORM > or use the Page Viewer Web Part, which supports HTML code.
< FORM > label. Learn more about the Page Viewer Web Part

I have the same problem in two site collections, no matter what HTML code I use. At the same time, on two other site collections, this problem does not exist.

More information. If I move this html file from _catalogs / master / .. to the Assets library, CEWP displays it quite well. But I've been using the _catalogs folder for HTML files for over a year without any problems, and I do not understand why all of a sudden this happens in multiple site collections. Old collections of sites with the same approach still work very well …

Has anyone encountered this strange bug? Any ideas how to solve this problem?