When I use generate noise, it sometimes removes audio from the other track I am working with.
I click a spot where I want to generate noise, and I see the top track grow because of the added noise…
…but then I also see the bottom track shrink because the effect removed something from it??? 
video example: https://youtu.be/kaCMghPdXo0
Windows 10 Pro 64-bit 1803
Audacity 2.3.0
Realtek HD Audio driver ver. 6.0.1.8555
Step by step, how are you generating the noise?
I select a position in the audio track that I want to generate noise. Then I generate, say 600ms of noise. I see the noise inserted in the selected track, but in the below track, a portion of the audio near the same timestamp I generated noise for the top track, is removed!
Are you sure that it is removing audio from the other track, and not inserting a space?
Look near the bottom of the “Tracks” menu, and ensure that “Sync-Lock Tracks” is not ticked.