Per IEEE Standard 802.11-2007 (emphasis is mine)
188.8.131.52.1 Sequence Number field
The Sequence Number field is a 12-bit field indicating the sequence number of an MSDU or MMPDU. Each MSDU or MMPDU transmitted by a STA is assigned a sequence number. Sequence numbers are not assigned to control frames, as the Sequence Control field is not present.
Non-QoS STAs, as well as QoS STAs operating as non-QoS STAs because they are in a non-QoS BSS or non-QoS IBSS, assign sequence numbers, to management frames and data frames (QoS subfield of the Subtype field is set to 0),from a single modulo-4096 counter, starting at 0 and incrementing by 1 for each MSDU or MMPDU.
You're right about this not being unique to v5.12. As I mentioned in my initial post, I tested with versions from 4.11 through 5.12 (now 5.14). Interestingly, I observe that a very old ROS 3.30, which I can no longer access for testing, DOES have working sequence numbers, so the sequence number feature was lost somewhere between 3.30 and 4.11.
I assigned the IP address to the ethernet port instead of the bridge because the address was assigned to facilitate configuration before I created the bridge. In production systems, I always move the IP address to the bridge. In this test setup, the IP address isn't used at all. However, I appreciate your taking the time to read through all of that configuration data!
My current testing seems to now point to a problem with station power save mode. Wireshark packet captures show the Mikrotik AP failing to buffer multicast packets while the iPhone is "dozing" in power save mode between Beacon frames. All of the other brands of AP properly buffer the multicast packets and send them during the DTIM interval after an appropriate Beacon flag indicating that sleeping stations should stay awake for a multicast burst.
Note that I'm not using WMM power save mode, which is an extension to 802.11. I'm using the "legacy" power save mode, which has been around for a long time.
I've sent all of the supporting material to MT support and I'm waiting for a response. I'll post results here as they come in.
If anyone would like to peruse the wireshark capture file, let me know and I'll send it directly. The capture file is too big to attach here.