If you spam the shield and then jump, it will cause a strong forward dash which is an insane movement exploit
ShieldExpansion-1.20.1-1.2.2 forge

Here's a clearer example of this occuring:

This is a recording using the Origins mod, specifically having the Avian Origin chosen, i think it might make the issue obvious enough
While not spamming the shield, this is just the user holding the button

It looks like the speed modifier is uncapped. If you look where it's applied in ShieldExpansionEvents#onStartUsing, you can see that a modifier is calculated and is multiplied to the player's original movement speed without ever checking for a cap.

I have a feeling that if you disable the speed modifier in the config, this bug will stop occurring.