Low oxygen with mask on but no leaks?

I'm just finishing month one with a BiPAP machine. Early on there was some adjustment in getting comfortable but things seemed to be getting better. Then 3 of the last 4 nights I woke up having thrown my mask off the bed. I don't remember taking it off. The first 2 nights I put it back on and went back to sleep. When I checked my blood oxygen on my watch, it had dipped to under 90 with the mask on before I removed it.

Last night I ripped it off after 30 minutes of sleep and checked my blood ox with a finger monitor. It was 85. My watch confirmed it was below 90 basically as soon as I fell asleep. Then jumped up to 95 after I took it off.

I tried troubleshooting last night and it seemed like there was a lot of air leaking from the hose nozzle or connection on the mask itself, and it seemed like there was no resistance at all to breathing out, almost like the air was being sucked out of me. Myair said the fit was good so I tried going back to sleep with it on. Woke up again with low oxygen and took it off and had it off the rest of the night.

I reached out to my sleep coach this morning and they're just saying the fit seems good and maybe I need supplemental oxygen. Which doesn't make sense to me because my oxygen is fine with the mask off. Even with the apnea my oxygen is much higher with the mask off the past few nights than with it on.

And there was a night in the middle where the fit felt good and I slept great. Does anyone have any similar experience or thoughts? The immediate suggestion that I might need supplemental oxygen is wild to me. They said they would forward my question on to a doctor and get back to me.