Th e way the distance is read for autofocus of SLRs is that some light is diverted from the focus screen and mirror to a discrete AF sensor. AF micro adjustment was a feature on DSLRs to compensate for errors of this process, particularly in lenses prone to focus shift, such as zoom lenses and fast-aperture normal and wide angle primes.
Mirrorless cameras do not have micro AF adjustment because the distance is read directly off the sensor and no such error exists.
BUT...
Backfocusing may still happen. Prior to the X-Trans III sensor, Fuji X series cameras were pretty notorious for backfocusing, for example. In the case of the Fujis, it was because the system was easily confused by high contrast vertical lines in the background. I don't know about specific problems with Canon R series cameras, but my guess would be either that the AF system could be focusing on high contrast vertical lines in the background or there could be a firmware bug. So try updating the firmware both in your camera and in your lens adapter. Then test the focus on high contrast vertical lines and see if it backfocuses.
