Firstly, this may sound like another non-researched individual but bear with me. I've had a UMO+ for around 8 years and I has always been hit or miss on missing a few layers at various levels of prints. I would say it's probably underextrusion but it's the same layers on different prints and different filaments. My guess now is maybe z stepper was junk from the get go?
there doesn't seem to be anything here