What Happens to Fixed Rate Mortgage Loans During Hyperinflation?

That's the risk the holder of the mortgage takes on when it agrees to a fixed rate loan. If inflation is lower than expected, the fixed rate mortgage is relatively more valuable. If inflation is higher than expected, the fixed rate mortgage is relatively less valuable.

For the most part, mortgage lenders in the US, EU, etc. do not hold the mortgage for very long. They generally sell the mortgage off either to the government (in the US, Fannie Mae, Freddie Mac, etc.) or to investors in the form of mortgage pools (you may have seen reporting on things like CDOs-- collateralized debt obligations-- in stories on the 2008 meltdown). At that point, the lender is insulated from the risk that hyperinflation makes the loans much less valuable and that risk is held by whoever acquired the mortgage.