Does exiling a card with flashback count as casting it for the purpose of spell mastery?

Spell mastery is a static ability that checks whether there are two or more instant and/or sorcery cards in your graveyard at a specific moment (usually when you cast the spell with spell mastery). It does not care about cards exiled from the graveyard — only cards currently in your graveyard count (CR 702.109a).

Flashback (CR 702.33a) allows you to cast a card from your graveyard by paying its flashback cost. When a spell with flashback resolves — or if it would otherwise leave the stack — it is exiled instead of going to the graveyard (CR 702.33c). This exile happens after the spell has already left the graveyard to be cast, so the card is no longer in the graveyard at that point.

Additionally, the act of exiling a card with flashback is not itself casting. Casting is the process defined in CR 601, which involves placing a card on the stack and paying costs. Simply exiling a card — whether as a cost, an effect, or as flashback's replacement effect — is never casting.

Example: You have two sorcery cards in your graveyard and cast a spell with spell mastery from your hand — spell mastery triggers. Later you cast one of those sorceries via flashback; when it resolves, it is exiled. Now only one sorcery remains in your graveyard, so spell mastery would no longer be active if you cast another spell mastery card at that point.

In summary, spell mastery looks at your graveyard's current contents (CR 702.109a), and exiled cards are in a completely different zone (CR 400.1). Casting via flashback removes the card from the graveyard and ultimately exiles it, which can only decrease your graveyard count for spell mastery purposes.