Thanks for all the details and for the file! The behavior does seem odd, but I’m not seeing the same issue on my side using version 5.5.
Could you try testing it on the 5.6 preview version, using a new Modular Control Rig on Manny, and let me know what results you get?
On my end, I only notice a minor difference in the shoulder area on both 5.5 and 5.6, which should be easily fixable with an additive layer on the baked Modular Control Rig track.
Also, I’m not experiencing the issue with the first frame being out in the animation sequence like you mentioned.
Backwards Solve on Modular Control Rig has improved quite a bit since its initial release, and we’re actively working on polishing it as much as possible! 