I had a quick look to see if I could single out the faulty procedure in Restunts, but I could not find it. I think the algorithm to calculate the bounding box of the explosion is still only in disassembly form; in the code ported to C the part concerning the explosion only kicks in when the coordinates have already been computed and converted to the screen reference system.
If you need something more sophisticated, I recommend intro_draw_text which does the job quite nicely (