Patch 3.1.0
"Changed Mad Grit such that no speed is gained or lost while slashing
and carrying a Survivor."
Source: https://deadbydaylight.gamepedia.com/Patch_Notes_3.1.X

this means that madgrit neither gives you a speed increase or decrease from what your base carrying speed is when using it (things like agitation will increase this speed) and is currently working as intended and is not bugged.

Madgrit was bugged within the freddy rework player test build were it gave momentum each swing however as stated above this was patched in the live release 3.1.0 and is no longer bugged.