Animation layer ignored at start of transition

In my base animation layer I have two states, an idle and a run.
I then have a masked layer set to override, which is synced and overrides both of these state's animations with the same "shovel equipped" animation.

The issue I'm seeing is that the Anim Controller will often get into a situation where the override layer is ignored at the start of a transition between these two states, and will be blended into during the transition.

In the gif I've attached I've set the transition durations to 1s to demonstrate what's going on. The layer is applied fine during the idle animation, then it's removed at the start of the transition and becomes effective over the duration of the transition. Note that the weight value of the override layer is at 1 throughout. I've attached the transition's properties if that's any help.

I'm going mad trying to figure out what's happening, so if anyone's seen this before and knows anything about it, please let me know!