Is the unused hot water line cooling all the other hot water lines?
Posting suggested test as an answer since the follow up identified it as a crossover.

When testing hot water flow and a crossover is suspected, an easy test is to turn off the water flow from the hot side at thetank to remove hot availability. When the hot tap is opened downstream, there should be no pressure, so very little water will come out.  If pressure seems normal or near normal such that water consistently flows without ceasing, it can only be a crossover OR a bad shutoff valve *(but if it was a bad shutoff valve, the water would be hot for a good while if provided from a tank).

At this point, its a matter of following the various faucets or mixing valves to verify their settings. These valves (mixing vales) can go bad, or be installed wrong, so it's usually worth it to turn off the cold supply at each suspect location until flow stops.

Congratulations on solving the issues.