My fp6 had a similar problem, for some reason it only displays correct charging times when pluged in to a high end charger otherwise charging would be like ten times slower so try plugging it in with a high end charger.
And the Problem is not that the charging times are wrongly displayed, but that the phone is only charged at 80% when the alarm rings - when it should be 100%
So, the phone correctly displays (in the evening) that it will be fully charged at 06:15, but at 06:15 it is at 80% only…
This may unfortunately be a FP6-specific bug: I have tested charging control on my OnePlus 6 today, and charging to 100% started around half an hour before the alarm.
The solution of deleting alarms and then rebuilding them sort of works? Sometimes I wake to a full charge, and sometimes it’s 80% precisely. And no, I don’t have charge limiting enabled.
I gave a reply 7 days ago to Report an issue and since that time done a little experimentation, not FP6. I seem to find a bit like you. My suspicion is that once you turn on charging control (whichever option) charging to 100% seems ruled out, but certainly not proven. That seems a “good thing” but also seems, the user actually lost control of the more important element “don’t cook the battery” by using the timed variant.
But users might like better information on what to expect. I prefer to use limit 85% and let timing sort itself out.