I have a solution to the groove problem! Alice from tech support looked at my logs and determined that “the B-axis value is too large”. Don’t ask me what that means precisely, but she recommended that I use the Luban slicer (I had previously used Fusion 360) and re-run the project. And that that did the trick!
I also discovered that there is an updated luban-fusion 360 configuration file available on snapmaker’s support site than what I had used:
Perhaps that fixes the B-axis value issue when using Fusion 360? Sadly I no longer have a Fusion 360 license capable of supporting 4-axis machining, so I cannot test that.
On a side note, while she was helping me debug my problem Alice had me test one possible issue regarding the rotary module. It looks like a slight variation of (or perhaps a more accurate description of ?) the solution provided to @CandleFX:
That turned out not to be the problem in my case.