Finally, I was exactly like you, I had setup my own custom lightboxes for login and signup and had setup the member signup settings as custom with existing login and with my login lightbox being the one in the custom choice.
i set mine up during December and like yourself, I had everything setup and it was working fine. Then all of a sudden it started to show Wix login window first again.
I complained to Wix Support and they told me first off that they couldn’t do it and I should go to Wix Code Forum instead, so I had to go back to them to remind them that it was Wix own settings in the editor that were causing it.
So finally Wix Support passed the details on to another department and I was told they would look into it and let me know when it was fixed. However, the stupid thing is after a good while waiting, I finally got another reply back and again they were saying that they couldn’t fix it as it was to do with my code and not the settings in the editor.
So after stepping back and calming down and drinking enough coffee to mellow me out, I finally resorted to fixing it myself. This is where I found others like yourself who had similar issues and about changing the settings to new member signup instead of leaving it on existing member.
However, it is pretty stupid and very obvious that Wix must have changed something somewhere to do with this area, as like yourself if seems weird that it was all working fine at the end of 2018 and then in Jan 2019 it suddenly starts being faulty.
To be honest I would not have minded if they had told us somewhere on new features or updates etc, that there had been a change to the way we had to code for custom login/signup options, or like with the member settings needing to be put on new member signup and not existing member anymore.
It just needed a bit of communication from Wix about it rather than just do it and not let anybody know and not expect users to moan, or if it was just an issue with the settings in the editor, at least to fix the issue within the editor itself and not leave us to find a workaround for it all the time.
Anyways, enough of my moaning now, I am going on too much!!
Onto our next problem solving mission…