If you disenchant boots with muffle on it, you learn the spell - works fine.

Creating an item with that spell, that item ALWAYS reduces MovementNoiseMult down to 0 when worn.
According to links in Creation Kit, it should be 0.5, not zero. Created a magiceffect myself and removed magnitude under spellmaking, linked enchants to it - then it works properly.

That means, muffle enchant will make you immune to movement detection.
No need for Muffled Movement / Silence perk (well, silence sets it to 0 too, make mufflemovement obsolete^^).

100% a bug. More enchants could be affected.

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Another one:

Let's say you skilled "Silence" perk, save your game, then install a mod which replace silence effects completly.
After your mod is loaded, new effects will apply AND silence effect is still active.

Other perks unload fine - there is something fishy around MovementNoiseMult.