I hear ya,
local machine shop told me these dropping seats is from overheating and they saw a few. Head and seat expand at different rate when heated right up.
As in your case, im sure there are other reasons it can happen, who knows. kms was low on mine and im only keeping it another year or so, so I just wanted to fix what was broken.
I also changed all 16 lash adjusters and rocker arms on the right hand existing head and the left (used/replaced) head. This was to deal with the possibility of any collapsed adjusters