Odds are your former landlord is simply waiting for the letter to come back. For reasons I am not in a position to explain, sometimes it takes a frustratingly long time for the post office to return undeliverable mail. In some cases, I've had correspondence returned as undeliverable several months after it was mailed.
I expect that the landlord wants to avoid paying a stop payment fee, or having two valid checks in circulation.
You write, "they didn't have an apartment letter" -- is that because your email with your forwarding address did not include the letter or was in some way confusing in relation to the apartment letter, or was it simply their oversight when addressing the envelope?

