How to register more than 5 simultaneous key presses? New Input System

I'm making a game where there are eight characters on screen at a time, each controller by a pair of buttons. One button jumps, the other "creates a gem." (For example, player one will jump if the F key is pressed, and make a gem if the R key is pressed. Player two will jump for D, gem for E, player 3 is S and W etc...) When I press all eight jump buttons simultaneously, all 8 characters will jump. When I press all 8 gem buttons, all 8 characters will gem. But when I simultaneously press some combination of jump and gem (more than 5 keys), it seems almost random what characters will actually perform the action.
I'm using the new unity input system, and have two separate action maps (same InputActions object). One for the first 4 characters, one for the rest. Each action map has a separate action per button, all of which are of Action Type Button. The two sets of characters are controlled by separate PlayerInput objects, but in my testing the grouping didn't matter, only the total number of inputs and if they were mixed between jump and gem.
Is this a known limitation? It's not on the "Known limitations" page.
If not, how do I get my game to accept more than 5 simultaneous inputs.
(Note: The only reason I think it is possible is because 8 simultaneous jump inputs worked just fine)