As the title suggests, with a fresh install of the modpack.
Wearing the Booflo Vest should offer a slam and multiple jumps, but does neither.
I have tried changing the controls to make sure it's not a control conflict and still nothing.

I did notice that with this pack, the jump mechanics are altered as there is now a fairly long 'Coyote Time' when walking off a ledge. Just adding as a note in case it's relevant.

Same thing for me, it does not work.