I have this issue whit Premiere 2017, 2018 and now 2019. I can insert/overwrite if i select two clips from project window, or it inserts/overwrite only the sound from source monitor window. Anyone with this problem?
The issue that you have described looks like an issue with the source patching wherein the source indicator for the video tracks might not be enabled. (Please check if V1 is highlighted for the source indicator on the video track)
You may want to go through this small video, this explains source patching and track targeting.
Source patching starts at 04:41.
Hope this helps.
Everyday you learn something!