I was pretty sure there were some structure blocks in the save, but I just reloaded the world and the issue has vanished (odd since it persisted through multiple reloads previously).
Strange.
attached world, chunk player is in is the chunk in question (3 -24)
No I did not. Structure blocks continued functioning in every way other than being unable to change mode, and this only occurred in 1 chunk.
This happens even when the stand is tagged Marker:1 which is a big problem for custom maps