When there is no block beneath scaffolding that was previously supported, it normally becomes unsupported one gametick later. Sometimes, it instead updates instantly (on the same gametick as the block underneath changing). The simplest way to demonstrate this is with a redstone clock with a 6-gametick period connected to a trapdoor with scaffolding on top.

What I expected to happen was...:

I expect the scaffolding to always update the gametick after the trapdoor activates.

What actually happened was...:
When an observer is watching the scaffolding, it updates instantly every other time the trapdoor activates. That is, half the time, it becomes unsupported the gametick after the trapdoor opens, and the other half of the time it becomes unsupported on the same gametick as the trapdoor opening. This behavior does not occur if the observer is removed, even if everything else is unchanged.

Steps to Reproduce:
1. Build a redstone clock with a 6 gametick period and connect it to a trapdoor. Put scaffolding on top of the trapdoor and an adjacent block. (See first image for a minimal setup.)
2. Use /tick freeze and /tick step to verify that the scaffolding behaves normally.
3. Place an observer watching the scaffolding on the trapdoor. There is no need for it to connect to anything.

4. Use /tick freeze and /tick step to verify that now, the scaffolding will alternate between updating instantly and with a 1-gametick delay.

Alternatively, build a symmetrical setup, where observers viewing scaffolding on trapdoors are connected in a loop. (See second image.) In this case, the scaffolding will not alternate its behavior; instead they will behave normally for several cycles until one of them begins to update instantly every time.