That issue is now probably fixed if you update the Jetpack mod to the latest version. It works by making a copy of your character to turn into a new jetpack character when fly (hence why there was a delay when starting to fly). It then copies back to the original character when you land.
The Amor Pockets mod updates the 'character_inventory_slots_bonus' value when you insert or remove a pocket from the armor grid. This mod is doing it correctly by setting the new value equal to the current value + pocket size. The jetpack mod had an issue which was just overriding the bonus value, not adding to it. That mod has since been updated to fix that and its better optimized so there isn't a delay when starting to fly.
If you still have this issue after updating that mod, then its likely a different mod overriding the bonus value, such as the "Hand Craft Priority" mod by SerhiiS. I've had to edit the mod to not override the value. For some reason it sets the bonus value to 0. I'm going through that particular mod to rewrite it to handle the bonus value correctly.
