Okay, so I can't seem to toggle the source track indicator labeled V1(depending on the video track). It is used to toggle the video track that you'd like to paste a clip to. In my case, to activate V1 to cut my MultiCam sequence. I have scanned my toolkit and preferences to see if it is disabled. What is also strange is that issue does not occur in other projects that I have done without MultiCam.
To add, I have recently migrated to another workstation and updated Premier from 2015 to 2017.
Source track indicator (V1) will disappear if you don't have any V1 media in your source window....
So considering you're trying to edit with a muticam sequence... perhaps your locator is at a part of the multicam sequence where there is no V1, (only V2 and/or sound). Try scrolling to a bit in the multicam sequence where there is V1 media. Or, load up a video only track into your source monitor and see if your V1 toggle comes on.