it sounds like you got the return line
instaloled in the slab.....that is not a good set up at
its best to eliminate that line in the floor and see what
happens, most copper retune lines will eventually start getting
pin holes in them, and in the slab you cant find them..
you could already have one in the floor and not know it....
if thier is a pin hole , it might make the water heater work
extra more and wear it out quicker..
at the very least put a ball valve in the line going into tthe floor
to isolate that run and see if you still get hot water to the furtherst
area of your home...
if you can figure out where it ties in on the other end it
might be prudent to cut that line out all togetheor