Dragon heads powered by redstone will stop opening and closing their mouth after about 4.5 hours (when the MouthTickCount reaches 327680), even if they still have redstone power. Turning the redstone power off and then back on does not fix the issue. The only way to fix it is to mine the dragon head and then put it back.

Example world (more than 10 mb): https://drive.google.com/file/d/1Vzpn-J-4ayY-kNyu2h6uquTe2oyvQReT/view?usp=sharing. It is at -136 69 27.

This has happened in multiple words.

If you ctrl-pickblock it, will not move even if you place it back.