I have not seen this specific problem before, but after some very strange and inconsistent behaviour, we tracked the problems we were having with Wix Forms down to the Safari browser. Our issues were more to do with setting up email notifications from forms, but when we tried it on Chrome, it all worked fine. Subsequent testing with Firefox, Opera and Vivaldi verified that our issues were Safari specific. We’ve also had issues with Safari and the Wix Blogs, so what we do now is work in Safari (our preferred browser) most of the time but switch to Chrome as soon as we run into strange behaviour with legacy Wix components - saves wasting a lot of time. This may or may not be your issue, but it’s worth trying another browser just in case.
Would be interested if anyone else has experienced problems when trying to work with legacy Wix components in Safari on a Mac.