Sorry about the loss of the two palmettos.
Years ago, I lost a Bald Head Island Palmetto after a 1-winter successful survival. The winter that killed it wasn't even all that bad!
On the positive side, I have a small Palmetto up close to the house that has been in-ground since 2007 or 2008. It's been defoliated a few times by winter cold, but is still hanging in there. The size is amazingly small. Picture a palmetto 2 or 3 years from seed in their native range. THAT'S my tree after 10 years! No trunk whatsoever. My same aged Minors are much larger.
Other palmetto growers have noticed this effect with their own trees. I call it a "Bonsai Effect." When palmettos are grown outside their range and are hit with cold the likes of which they are not accustomed to, they can either die, defoliate and fully recover, or, defoliate and recover, but in a permanently stunted form. It's as if a palmetto morphs into a Sabal Etonia. Permanently. This has been observed as far south as central/northern Georgia. The plant can push out healthy green, pretty fronds, but it will refuse to grow vertically. Very interesting phenomenon.