If you ever tried to find this fix, the internet is pretty much dry since Medieval 2's kinda dead. I found his fix, albeit an older version in an old mod.
Unzip everything, then place the files in your en_pplate_plate folder in unit_models.
When you go into the game, your bucket helm equipped heavy infantry will now have their necks fixed.
All is good with partial plate infantry, however the full plate infantry now gains a weird studded tunic over their plate. It still looks good, so it doesn't bother me. It adds a little more variety than it was prior.