I'm experiencing the same thing and I have been for a number of weeks. Almost certainly as far back as 2/16. For some items it shows the total quantity I own, others it doesn't. I haven't noticed any commonality among what it shows for and what it doesn't. In fact, it changes. Sometimes, not all the time, when I move the slider to increase/decrease size, it will change which ones it shows quantity for and which ones it doesn't, but not in a predictable way, from what I can tell.
This behavior continued to persist after performing a clean installation as suggested, by following the instructions exactly as outline here:
https://help.mtgo.com/hc/en-us/arti...stall-or-perform-a-clean-installation-of-MTGO
Including the system restart and all.
The only consistent way to see the quantity of what I own is to switch to list view or to add the entire stack to a trade binder.