The question "Can I use spell slots to cast the spell granted from the Fey Teleportation feat additional times?" asked whether a character can use a spell slot to cast a spell "learned" through a feat such as Magic Initiate or Fey Teleportation. The answer, in a nutshell, was: Yes, but only if the spell is on the spell list of the class providing the slot. So, for example, an elven wizard who learns misty step by taking the Fey Teleportation feat could use wizard spell slots to cast misty step, because misty step is on the wizard spell list.
A piece of the answer's rationale was the following guidance from Sage Advice Compendium v2.2, p. 8 (emphasis mine):
If you have spell slots, can you use them to cast the 1st-level spell you learn with the Magic Initiate feat?
Yes, but only if the class you pick for the feat is one of your classes…. In short, you must follow your character’s normal spellcasting rules, which determine whether you can expend spell slots on the 1st-level spell you learn from Magic Initiate.
Let's imagine a multiclassed Rogue 3 (Arcane Trickster)/Wizard 2. According to the spellcasting rules for multiclassed characters, see PHB p. 164-165, this character would have two 2nd-level spell slots but would not — at least not through his class levels — be able to learn any 2nd-level spells.
Now imagine that this character gains a level, becomes a Rogue 4/Wizard 2, and takes the Fey Teleportation feat. He "learns" misty step, which is on both the the wizard spell list and the Arcane Trickster spell list (because the latter is really the former). He still has no access to 2nd-level spells through his levels in either class.
Can this character use spell slots to cast misty step even though he couldn't otherwise cast, or even learn, any 2nd-level spells?
Best Answer
No; you can only use slots to cast lower-level spells from your class
The PHB rules (p. 164) for multi-class characters specifically address a multi-class character with slots of a level they can't "know or prepare". And it says You can use those slots, but only to cast your lower-level spells. It further elaborates that if any of those lower-level spells are improved by being used in a higher-level slot, you may get those benefits.
If this loophole worked, a similar loophole would exist for wizards: they can only prepare spells in their spellbook. And the rule for whether they can put a spell into their spellbook is only that the spell must be 'of a level for which [they] have spell slots'. So a multiclass wizard with an up-level slot can find a higher-level spell, and then could copy it into his book, and then prepare and cast it. RAI is pretty clear this won't work (after all, RAW explicitly says it won't, it just also kind of contradicts itself by implication).
In other words, the multi-class character rule saying you can't overrides normal rules for learning and casting spells. It is the "more specific" rule. The character may not 'know' or 'cast' spells higher level than their class levels allow. Which means your example wizard can't copy misty step into their spell book; that would constitute 'knowing' the spell. Nor could they prepare and cast it.