Cubase always “thinks” in beats, not bars (in other words, changing time signature does not move the position of events).
In the example you used (I presume that the whole project was originally in 4/4, before changing bars 10-19 to 3/4?), bars #10 to the end of bar #19 lasts 36 beats (i.e. beats #41 to 76), and the beginning of bar #20 is beat #77.
If you change bars #10-19 to 3/4 (and put it back to 4/4 at bar #20), they now last only 30 beats, but the “old” bar #20 is still at beat #77 which has now become the 3rd beat of bar #22. (and that isn’t a “quirk” of Cubase… it is simply what happens in music )
So, yes, you have to move those "old "bars #20-28 manually. Just select everything from what-is now bar #22 beat #3, and drag it to the start of bar #20.
[EDIT: even if you use “Process Bars” ]