In a custom state, when you start a movement with a set speed on a ground that alters walking speed (such as holes or ladders) while having done state:set_affected_by_ground(ground, false), it applies the slowness to the movement's speed instead of keeping the full set speed. However, starting the movement on a normal speed ground (traversable), then going over that special ground does work as expected (the initial speed is retained during the whole movement. This is especially true with the feather item from the ALTTD project: shallow water and holes (the two I encountered the bug with), despite having been set as ineffective in the jumping states, do their slowness effect, but only when starting from said grounds, mostly resulting in falling into bad grounds.