I just upgraded to 3.1.2 on Catalina and it gave me a gatekeeper dialog and wouldn’t start.
The workaround is to find it in the Finder, right-click on it and select Open and then tell it to open anyway.
We can’t have nice things. 
(to be clear, I’m blaming this on Apple moving the goalposts with every release)
I haven’t encountered that issue. However, that is a security measure that will appear with any third party application not acquired through the App store. It is a slight frustration but a worth while one. After you do it once you shouldn’t have to do it again.
Yes and no. If all the i’s were dotted and t’s crosses properly, the first gatekeeper dialog would merely ask you to confirm opening it. In this case, it says that Apple cannot prove that this is not malware and refuses to open it. You have to go find the app, right-click on it and open “anyway.”
I believe the shortcoming here is that the app was not notarized - a separate operation from signing it.
I’ve been through this exercise as an Apple developer.
Ah I see, I misunderstood. That is frustrating. I just updated to Catalina and haven’t encountered that issue…yet 
I believe during the upgrade existing applications are grandfathered. If you upgrade PreForm before they notarize it (and if they notarize the binary they don’t have to re-release. The notarization is actually an online thing and doesn’t change the file), you’ll hit this.
The workaround is easy, it’s just not obvious, which is why I posted.