I took a quite look at your recent ride (hope you don’t mind). There’s nothing out of the ordinary happening here. Phew, right?
It might help you to understand what is happening to your trainer in ERG mode as you go from high intensity to a low intensity interval. TR is sending a message to the trainer to sit at a prescribed wattage (which changes based on interval needs). Your gearing is giving some resistance and the trainer is giving some resistance. Both are controllable. You can change your bike gearing to give more or less resistance. If you give it more resistance, say by changing from the small to the big ring, then your trainer will compensate by letting off some of it’s resistance until it matches what TR is telling it to be at. So, if your gearing is fixed (mostly it will be if you’re in ERG), the slackening in resistance when going from high to low intensity intervals is the trainer catching up with the change in resistance which can take a few seconds.
To some degree, how long this takes can be controlled by you. If you have high gearing resistance (big ring up front and small gear at the back) then your trainer will have less resistance and the flywheel will be spinning very quickly and have a lot of inertia. If you reverse this equation and have less resistance on the bike (small ring up front and bigger cog at the back) your wheel will be spinning slower and the trainer will have more relative resistance. When the change down in intensity is called for by TR, your trainer doesn’t has more resistance to make the change with and less flywheel inertia to fight against.
@GPLama also has some really excellent videos that probably better describe this if you’re still scratching your head.
Hope this helps!