I took a look at the hak and to fix the texture issue you’re seeing - for the normal mounted human male phenotype - simply delete all plt files with the pmh3 prefix from said hak. pfh0 / pmh0 plts are enough for all phenotypes and races. Therefor I don’t think this is related to any NWN update.
I looked a little further into it and you were right, this issue was caused by update 8193.32. Under Other Features in the patch notes it says:
PLT textures can now be phenotype-specific.
So those pmh3 plts were borked from the start but never caused an issue because they were ignored before. Since that patch the male human normal mounted phenotype (aka pmh3) used those specific plts and therefor the messed up texture issue.