This would probably fit better as a VS Feedback item, but since this is the only place I have found someone referencing the same issue, I thought I would add some confirmation. I do not have a solution, only more examples to add. This issue has been annoying me for months! It seems to happen most often when typing razor code inside of JavaScript, especially inside of a quoted string.
Here is an example from a test project, where I am attempting to put a reference to a model field into a JavaScript string. Notice that I click tab a couple times and it doesn't actually insert the intellisense result at the point I am typing. But when I scroll down you can see it inserted it randomly in another spot pretty far away!

I did also try to remove the hidden .vs folder, bin/, and obj/ and could still reliably reproduce the issue after restarting Visual Studio. Currently on 1 before the latest (16.9.4). I did notice that if I re-open this cshtml file, wait until the file seems to be fully loaded (all the red errors go away), I also reliably get an ArgumentException that occurs as I begin typing. Then when I scroll down I already see that Model has been randomly inserted in another spot in the code.

I'm going to try updating to latest (16.9.5), and starting in Safe Mode to skip loading all 3rd-party libraries, and see if the issue is still reproducible, in case it is caused by one of the handful of extensions I am currently using.
Edit: Well same issue with 16.9.5. Trying to use Safe Mode is a no go, it makes the intellisense and syntax coloring completely disabled for .cshtml files.