The fault of the BM869 in all ACV / DCV modes can be confirmed and also quantified:
As the peak voltage (AC + DC) exceeds about 1.095V, an internal amplifier clips the signal without overload indication, and the TRMS circuit / software creates increasingly high measurement errors, for both the Udc and Uac components of the signal.
I made a measurement sequence, using an HP3325B signal generator for ACV plus DC offset at 60Hz, and the BM869 only.
I configured the DMM to 5V Udc, DCV and ACV dual display, and the 500mV Udc range likewise.
On the 5V range, the DMM would always display correct and precise values, checked by the HP3458A in DCV, ACV and DCV+ACV mode.
I compared the 5V range readings with the related 500mV range readings, and calculated the % error between them, and the resulting Vpeak.
Errors of about < 0.2 .. 0.4% between both ranges indicate correct readings; this limit is due to the 10 times smaller resolution of the 5V range.
Every error above that limit indicates the described clipping effect, which sets in sharply at about 1.10V.
In the extreme case of overloading with 0.56Vdc, the DMM still displays no OL, and a really big error of -15% occurs.
As you see, the battery was already a bit low at 7V, and 6.2V with lit display.
This did not affect the accuracy greatly, though I turned off the light at the crucial limit values.
This 1.095Vpeak limit is valid as well for a fresh battery @9.3V.
This effect will also appear on higher ranges, I assume. So that's a really bad characteristic of the BM869, to create such gross errors w/o any warning or OL indication.
Anyhow, an unknown DC/AC signal can always be analyzed correctly by simply switching to the next higher range.
Frank