Not a regression: neither release has it. src/mixnet/stream/mod.rs is identical between 1.21.5-rc.3 and 1.21.5, 504 lines in both, and send_to_stream starts at line 125. The branch I mean is the else at line 165, where a frame whose stream id is not in the map falls through and nothing is logged.
On develop the same file is 731 lines. It gained an orphans map beside streams, a buffer_orphan that holds frames whose stream has not registered yet, ORPHAN_TTL at 5 seconds, MAX_ORPHAN_STREAMS 64, MAX_ORPHAN_MESSAGES 32, and a register_stream that drains whatever was held for that id. In the monorepo it is sdk/rust/nym-sdk/src/mixnet/stream/mod.rs, and it came in with #7057.
So my expectation was wrong, not your release. Your description of the process answers it: it is on develop, develop becomes the next release branch, and the fix rides that. That is all I needed for the pinning decision. We stay on 1.21.5 and the probe keeps covering it until the next release lands.