Are you sure you are using it correctly? In the ‘B-52 Driver’ FLIGHT MANUAL Part II – Aircraft and Systems (csb521_manual2.pdf), page 47, has this to say about the Nav Mode Selector Switch:

The rotary switch has TACAN-VOR-ILS-ILS APP positions.
· In TACAN position, the bank steering bar will respond to the MSFS flight plan (if loaded).
Note: Nav Mode Selector Switch is active only if Heading Selector Switch in NOR position.

I'm not sure how you are trying to use TACAN mode, but as you can see, TACAN mode only follows an MSFS Flight Plan.