This has turned out to be a hardware failure. Mikrotik support determined this when I supplied a RIF file and analysis of it showed "FCS errors" on the ports in question. I've contacted the vendor since warranty exchanges are handled at that level.

Support has replied and examined a supplied RIF file since I posted this. Here is the response: > There are FCS errors accounted on both ether10 and ether11 ports, they are usually caused by cable or port damage. To be sure, please, recheck the connection with different cable and after switch reinst...

I posted "CRS-328 slow ports in default configuration" (https://forum.mikrotik.com/viewtopic.php?f=2&t=171433#p838223) and it seems like a similar issue except that it's present on 6.48 stable and the CRS328 has a Marvell-98DX3236 rather than a Marvell-98DX3257. Can anyone help me spec...

(A note to mikrotik employees: the cause of this seems to me to not be a configuration issue so I've filed an official support request as SUP-38183, previously SUP-35938. The request has been unanswered for over a week so I am posting here.) I've got a CRS328-24P-4S+ with 4 suspicious ports. I've up...