After a lot of tinkering it seems that I have found a temporary solution to this bug, maybe this knowledge might be useful to you.
When you do convert the control rig to additive, its all messed up due to some offset values adding on top after the rig is applied (in the rig window they are fine for some reason, seems to be something in the editor. But you can reset the controls` transform manually in the anim details.
The controls you need to manually reset in sequencer:
- Clavicle controlls
- Neck01
- Ball controls
If you switch to IK arms they will be broken too, but resetting IK control transform and rotation does fix it too. If you switch back you may notice some issues but the reset for all FK controls fixes it too.
Screenshot with that little arrow near the transforms that resets them:
So, basically if you are using it for linear content creation like me, its the easiest way to make it actually work in 5.5.1
Screenshot of this rig in a working state: