Oddly enough this is something i never noticed before.
The Drakk Cortex Hybrid has a racial slot where shields can be equipped into. However, this racial slot does not seem to allow fast-charge shields to be equipped at all. Attached save is a quick example of this quirk. The character has 4 shields in its inventory (2 fast charge, 2 regular). The regular shields can be equipped, the fast charge shields cannot. I haven't tested if this applies to other shield sub-types as well.
Savegame and Character files:
WeTransfer Download Link