Good day @venharhaber. I am also using the jQuery Migrate Helper plugin to roll-back the JQuery library to a pre WP 5.6 version. This was to “fix” the problem described in my ticket https://core.trac.www.ads-software.com/ticket/52111.
I understand that it is only a temporary fix and is masking the underlying problem that the Classic Editor (and it’s implementations) are using unsupported and deprecated JQuery calls.
There is a similar (although MUCH more detailed and better written) ticket to mine – https://core.trac.www.ads-software.com/ticket/52133 – that is currently marked as “has-patch” and slated to be incorporated in WordPress point version 5.6.1. This is due to be released in “mid-January”.
I suggest you keep an eye out for this release, change the JQuery Migration Helper JQuery Version setting back to “Default from WordPress” and see if you problem has truly been cured and not just masked.
Oh, and, I imagine (but I may be wrong) as the JQuery Migration Helper plugin is logging ALL calls to the JQuery Library this may well slow down a busy website.
Hope this helps!
Spike