I cant say if anything is wrong with your BT. Maybe he feel comfortable at that location.
Is it pretty new addition to tank? what are its tankmates?
Is it showing any patches of white or paleness in coloration in the face or along the lateral line on either side/both? Is it eating well and have full tummy?
DO you have
FOWLR of Reef Set Up? If either, I strongly doubt any problem with water issues but how about posting the results?
Many times, I have added well cured (by me)
LR and fishes at the same time w/o consequences but wouldnt recommend until you feel comfortable or have better understanding of what LR can do as I have learn by dealing with uncured/cured LR for just about 20yrs.
Sorry for all the questions but just trying to understand your tank.
I've seen such behaviors (some by intake tube or by other equipments) from other tangs including BT but usually to avoid confrontation with its tankmates.
Hope all is well!.