Alright, I think I found the issue. I opened a PR with a fix. Line 512 of monarch.lua:
screen.transition_url = screen.factory_ids[screen.transition_id]
…but earlier on line 325, it’s set to nil because screen.transition_id is deprecated.
PR here: #100
ps. I am not at all familiar with the monarch source so this might not be the best way to fix this. I just did this because it was low-touch and maintained the “backwards compat” for that deprecated functionality and fixed it moving forward at least for me.