This appears to be in most everyone's game, though it only appears on two conditions:
1. the castle is attacked
2. using a settler Bell at The Castle
the Radio NPC stands up, moves to either the bell or to defend, but is unable to return to his seat.
the only solution I found is changing the setpapyrusvar ai.

At any rate, this batch file operates on using a one-time easy to input console command to either never have this happen, or allow you to fix the bug once it's already occurring.

If you have downloaded and are intending to use it, just type; "bat 1, bat 2, bat 3, bat 4" into the console and it'll run through 5 commands that fixes the ai, allows you to assign Radio NPC to the radio transmitter if necessary, but with further testing, he goes back to his station on his own so that shouldn't be needed, just an additional possibility so he doesn't require any further fixing, at that point you could command him to go back to the receiver like any other settler you assign to harvest food.

Apply other tags if you wish, I might upload further quest/quality of life bugfixes as they happen, mainly uploading this to allow people experiencing it a direct link to fix it without telling them to input the console commands themselves.
Any large bugfix people are free to add this to their own collection, adding credit to http://www.nexusmods.com/fallout4/mods/4831, I didn't know it was on here prior.