It's a very simple mod that should work as intended, so make sure to have exported your mod order through your manager and check for conflicts with any other mod that modifies the same spell.
Mod order's been exported and rearranged many a time to try to fix it, to my knowledge I also have no mods that would modify Expeditious Retreat specifically, unless 5e spells does something wonky unbeknownst to me
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Yeah, that's probably it.