Threading fit issue, tapered?

[tobnpr]

Did you take any spring passes ?

I keep coming back to this. If there's minimal stickout/workpiece isn't deflecting, and the turned down tenon OD is consistent along its length, I don't see how it could be anything else.

 

[Ischgl99]

The thread mikes are sort of measuring a combination of both because if the thread pitch of the mike is slightly different from the cut thread it will not fit all the way to the bottom?

Thread mics measure on the pitch, not the bottom of the thread.