Losing Energy Shield when swapping weapons

When swapping between weapons I expect to stay at the same % of Energy Shield (regardless of values).

So, when I have 100% Energy Shield, after swapping weapons I should still have 100% Energy Shield. (Even though the absolute numbers might differ.)

However, when swapping from a bow (low ES) to a focus (high ES) I essentially lose all the "extra" ES that I should get. You even see the screen turn red as if taking a lot of damage.

This makes weapon swapping between a focus and a non-focus very bad. Every time I swap weapons, I lose 50% of my total energy shield, and then I have to use a potion (or recharge) to heal back up.

During a long boss fight, this means that pretty much all of my Flask charges are used to heal from the damage I take when swapping weapons.