Oh my god, you’re such a lifesaver. It makes sense a timeline works, because I think the problem is how the motion blur is calculated, if the mesh vertices change position gradually over time then the motion blur will be almost zero.
If you change the value from lets say 0 to 1 in a regular function call the vertices change position in a single frame, that causes massive motion blur to occur and it somehow gets locked in the blurred state.
I wanted to change the value instantly to 1, I found that adding 3 keys in a timeline of .25 seconds works perfectly first key is at value 1 second key at value 0.99 in the middle and the last key again at 1. This makes the change from 0 to 1 instant and causes zero flickering.