Sorry, maybe I misunderstood you, but I don’t get it.
I just tried to provoke a situation with your “issue” of wrong AC readings, but I couldn’t – provided the measuring mode is adequately set to “VOLTAGE AC” (as said).
If I naively try to measure mains voltage (~230 V AC here) with all settings at their defaults (that is, wrongly “VOLTAGE DC” selected), then of course I get a nonsense reading of something like 19.xxx V or so (while sampling is auto-set to 125 Hz / 64smpl). But if I switch to the correct mode “VOLTAGE AC”, the sampling automatically changes to 4000 Hz / 256smpl, and the correct voltage is displayed immediately.
This is surprisingly even the case when logging is active. Although we have been told that autoranging is disabled while logging, the sampling rate seems to be still automatically adapted if left in the default setting “AUTO”, thus ensuring correct measuring.
So I really can’t see your problem. (But it’s not important, maybe others can.)
BTW, it’s somewhat offtopic in this context, but I found another real strangeness (if not a bug) during the above test: If I set CH1 to “INTERNAL TEMPERATURE” while CH2 is wrongly set to “VOLTAGE DC”, then in the moment 230 V AV is connected, CH1 stops displaying temperature and changes falsely to “OUT OF RANGE”. If I switch CH2 to “VOLTAGE AC”, CH1 goes back to normal temperature display. WHY? It’s very irritating that CH1 is influenced by things happening on CH2. Shouldn’t both channels behave completely independent?