2. It would be interesting to test this with all non-Toolset plugins disabled and a default theme like Twenty Twenty.
If it's fixed, you can start adding the disabled items, one-by-one, to narrow down to a possible conflicting one.
3. In case the issue still persists, I'll need a clone/snapshot of your website so that it can be investigated on a different server, without affecting the actual website.
( ref: https://toolset.com/faq/provide-supporters-copy-site/ )
I've set your next reply as private and please let me know how it goes.
Thank you for your direction on this - I found and fixed the issue. I'll update you on the other issue from which this was split off as soon as I can implement and test. I appreciate your detailed explanations!