I had an even closer look at the noise suppression of 5M and 50M range.
It's possible to greatly suppress this 50Hz noise on the 5M range by a parallel 100nF MKP capacitor, even with a strong magnetic 50Hz field of a transformer inside a lamp, see picture:
The 'normal' noise can be suppressed so far, that you get an absolute stable reading, only a few counts fluctuation..
The 50MOhm range, though, is absolutely stable, it even shows no static shift by the noise, because the 100nF capacitor makes no difference.
In contrast, my BM869 shows not that kind of noise as in the experiment above, but in both 5M and 50M ranges, a static upwards shift is observed, when the 50Hz magnetic field influences the cables.
This shift can be completely suppressed by adding the 100nF capacitor.
In the schematic of the 121GW, there are range reference resistors R13 (1.11M) and R29 (10M) and a 27nF capacitor, C30, tied together on one side, and towards the network MUX on the other side.
I assume, that at first, the 50Hz noise filter is not correctly programmed, at least in the 5M range.
Then I assume, that this filtering capacitor C30 is switched on, parallel to the 10M reference resistor, so that even this static shift effect is suppressed.
In the 5M range, I suppose, this C30 is not switched on, in parallel to the 1.11M resistor.
In the lower ranges, either the noise filter, and/or this suppression capacitor might be correctly configured.
The 50 Ohm range is unstable on the last digit, due to another reason, as the 100nF does not change that behavior.
So I propose to UEI, to check the digital filtering, and also propose to switch on the 27nF capacitor in the 5M range, if that is really the method used on the 50M range.
Frank