Inline Form work sporadically - but never in Safari

I've got several Constant Contact inline forms on my WordPress website. There have been recent complaints of the forms not working. After a lot of testing, we've discovered that the inline/embedded forms: 

1. Never worked on Safari (18 of 18 test submissions failed)
2. Sporadically worked on Chrome (about a 33% fail rate)
3. About a 60% fail rate on Edge

I have a Windows PC and can't test on Safari - but personally have had zero problems (so far) with Edge, FireFox and Chrome.

As far as I know - I have the correct universal code included.

So my questions are: what would cause the inline/embedded code not to work at all in Safari? Are there known problems (and fixes) to the same inline/embedded forms working in other browsers? I recently added the Constant Contact plugin to the site in order to add forms in an Elementor PopUp --> could that be interfering with the inline/embed functionality?