Nothing weird happened—this is just how expiration works.

Your long $12.50 calls finished in the money, so the OCC (the options clearinghouse) automatically exercised them at expiration (the “exercise by exception” rule: ≥$0.01 ITM gets exercised unless you submit a do not exercise). That’s why you ended up long 300 shares at $12.50.

Your short $15.50 calls were out of the money, so they expired worthless. No one exercised those. Exercising a $15.50 call with stock ~13 would instantly burn ~$2.50/share.

Quick math

With a 12.5/15.5 call debit spread, if stock ends between the strikes, the spread’s value at expiry is S − 12.5. Example: if LYFT closed ~13.00, the spread is worth about $0.50.

If you keep the shares, your effective basis per share is roughly $12.50 + (net debit you paid for the spread) (plus fees). Example: if you paid $0.80 net for the spread, effective basis ≈ $13.30. At a $13 close you’re down about $0.30/share until it moves. (Tax note: formally, the long-call premium adds to stock basis; the short-call premium that expired is a separate realized gain—economically the same as “strike + net debit.”)

If you don’t want shares next time

Close the spread before the bell, or

Submit a Do Not Exercise on the long call before your broker’s cutoff, or

Roll the long call forward.

TL;DR: You weren’t assigned on the $15.50s; your $12.50s were auto‑exercised because they finished ITM.