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But please do repro this issue on your end and respond. This is a very common issue with my users too. There is not predictability.
I think it's hard for level 1 support here to repro an intermittent bug like this. From the many reports about it, it's clear it's not related to check out. Most likely it's another symptom of Msft's lack of expertise in the cloud storage space - I don't think they've figured out co-authoring and a whole host of other authentication issues properly yet. So this needs to be escalated to a higher level that can dig into the why of why it might happen. Since that won't happen here, I suggest everyone who experiences the bug submits a service request so more competent engineers are made aware of the problem.