Hi. Oh, that awful sinking feeling! Actually I can’t think of any reason why your bias-T got hot. The internal bias T in your RSP will only supply 100ma before the safety cutout operates, which isn’t enough to cook anything much.
There shouldn’t be any DC coming out of the external bias-T’s ‘radio’ port, it is DC blocked to stop any nastiness like this. So why did the bias-T get hot? The good news is your RSP and its bias-T seems OK.
The only reason I can think of for the heating-up is that perhaps you had a short across the loop’s connection. Obviously it’s not there now as it works. A mystery!