I have a similar set up, but all my filters are supposed to be "par-focal", and while the offsets are actually a little different from each other I don't lose guiding when re-focusing (I tend to use a low max-travel, not too high step size etc...).
It may also be that your OAG camera is at the limit of focus when your main imaging camera is focused, try checking that the guide camera is par-focal via the OAG to your main imaging camera?

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