I think I may have found a workaround that I am currently testing. The problem I was facing was that mainstage didn’t recognize the beat one. So what I did was instead of syncing the tempo (might also work if you synced the tempo) I just set a fixed tempo in the concert or patch. I then just had a midi cc message send from the device that I wanted to bring Midi into mainstage trigger the playback button on mainstage. When you hit the playback button on mainstage it pushes the internal clock to beat one in mainstage internally (in theory). So mainstage recognizes that when you hit playback that is the start (beat one) of your tempo.
So my setup is that my drummer has another instance of mainstage running on a separate laptop that runs click and other instruments/ tracks, but I wanted my laptop - that has my synths, to sync to that clock/ computer. So when my drummer hits space bar on that laptop, it starts the click and then also sends a midi cc message through a midi cable to my computer that tells my mainstage to also start playback (the play button) This in turn, starts them relatively at the same time and on beat one. Then when the song is done he hits space bar again and that sends a midi cc message to my computer to stop playback. It isn’t completely full proof, but it has seen better results than the alternatives I’ve tried. I’d be interested if someone tested this out, or used this theory to make it better.