For some reason this happens to naturally generated doors (ex. doors in villages, strongholds, etc.). I figured this was an Optifine issue, but I've run the same version of Optifine on other versions and have not had this issue. I then assumed it was the 6.5.1 version of MalisisCore, which apparently is considered a snapshot. Perhaps that is the issue?

in the malisisdoors.cfg config file setting B:config.modifyVanillaDoors to false fixes the problem

Yeah, though that messes with player-placed doors, which I don't want to do. Hopefully there can be a fix, though this is indeed a workaround.