Hi Nigel - thanks for your reply, which I can now see I misinterpreted and thought that your developer wasn't going to fix the problem rather than seeing your intended meaning of the instructions in the other post wouldn't help in this situation, which I can now confirm having tried it on a local copy of one of my websites. The only thing which gets rid of the issue is removing the conditional rule for the display of the custom field group.
In my case I can reassure that activating a standard theme and then reverting to my original theme does not brek the website.
Also the message doesn't appear to cause an issue if I ignore it.
It is safe to simply ignore the messages. The text of the existing message should more accurately say "Conditions for displaying some custom fields on this page might have changed. Reload this page to see the changes."
The developer plans an update to only show the message when they actually have, but I don't at the moment have an ETA.
I'll update here when I have something more specific.
I'm just doing some house-keeping here and note that Types 3.3.7 with the fix was released already, and if you have updated you should have found the problem resolved.