You should report all bugs and errors to LW's support.
http://forums.wolflair.com/showthread.php?t=24976
However, don't expect an update from them. I'm sure you're aware that they haven't released a patch since Apr 2019. Their own announcement on the HL Online forum states they are focusing their development on their online programs. We probably won't see anything new on HL Classic until they release their HL Online version (2021 if at all).
I'd recommend using ShadowChemosh's Community Pack to try to manually correct the error.
http://forums.wolflair.com/showthread.php?t=51966