Setting NPCs to use something (be it walking speed, or clothing overrides, or FNIS Sexy Moves, or neck height or forward/backward head position or other skeleton changes via Racemenu) seem to fall back to installed values after a while. I've found it best to adjust only the player, or accept that they won't hold the values I've applied to them,
Even followers don't. Or at least some don't. Especially the older ones like Yuih Sha (circa 2018), which, even after swapping out the old UNP body for BHUNP, won't hold an FNIS sexy moves setting through a cell change, or for that matter, if their heights or other skeleton values are changed in Racemenu, they won't hold those permanently. I presume it's something in their esp files, but I'm not intimate enough with those to figure it out and solve permanently.
As for player speed....Mine will sometimes violate "the rules" and seem to do double time in sneak mode.. I presume those are due to some glitch in what's supposed to be a temporary speed change when walking or sneaking (or some other movement...). For instance when sneaking, if you equip a weapon while in sneak, the travel speed increases until you stop or disarm your weapon, (if you have achieved the perk ~~"faster sneak with equipped weapon"), that can get glitched, leaving you in speed mode once the condition is no longer true - not sneaking, or weapon removed, or both, or when walking and equipping a sword, your stance get altered while the weapon is equipped, which can get stuck even once you've put the weapon away, or you sometimes find that you can't use a weapon after jumping off a precipice when the weapon is equipped.
I can usually solve all of these by going to 3rd person and jumping (fwiw). That is, if I notice that's something wrong. With walking speed, such anomalies can be a bit harder to detect unless it's accompanied by an obvious stance change (like the sword stalking walk vs normal walk), when the change is more subtle and only fractional (ex: walking speed should be 1 >>> IS 1.15, etc)