Not sure if anyone else has come across this issue. I am using Realtek and I can see there seems to be a lot of stuff about Realtek drivers here but nothing that seems to relate directly my issue. To be fair, it might not be a Realtek/driver issue.

When increasing the App Volume slider one point at a time by using the arrow keys it stops at 52 and at 58 because the next value doesn't seem to exist.

Also, maybe related, the volume increase doesn't steadily increase from 0 - 100, there is a slight drop in volume between 24 and 25 and 58 - 60 and a larger drop in volume between 49 -50 and 74 - 75. The loudest volume setting seems to be 59 for some weird reason. I only have Dell machines to test on with Realtek audio so would be grateful if someone could let me know if they experience the same issue (I've tried in a VM and get the same but only on a Dell machine with Realtek)

I've tried it with the same results on W10 1809 and 1903