FindRTXPayloadType compared the whole fmtp line with "apt=<pt>", so an
RTX codec negotiated as "apt=96;rtx-time=3000" (RFC 4588) was never
found. The down track then stored RTX payload type 0, sent
retransmissions on the primary SSRC and probed with padding even though
RTX was negotiated.
Compare only the apt parameter of the fmtp line.