In all these cases the UI kept working OK?

Yes.

Did you ever get any sort of failure in the UI?

No.

Example: missing data, page load failure, etc?

No. nothing, I don't remember if i clicked on some button or link , or just the stack trace arise by doing nothing.

Trying to catch this again ( i have no idea how, but trying )

For now, I noticed that from time to time this is executed:

200 OK

apiman [9:04:28 AM]>>  Bearer token successfully refreshed: {
  "token": "eyJhbG ... long token here",
  "refreshPeriod": 240
}