This is very odd. I've resolved the issue, but don't know how I did it.
Spoke with Apple Support. After a half-hour of chat, was elevated to a senior tech. We spoke on the phone; he shared my screen, helped evaluate the MBP. We transferred a large file from the MBP to the external SSD—which I had to unpack, as it was ready to be returned.
No problem with the transfer. Hm, that's good. But not really much of an indicator of anything is it? I mentioned that I had Blackmagic on this machine, asked if that would be of use. He agreed. I ran it. And to my shock and amazement: 2100MB/s. Huh?
Tried it repeatedly on each of the ports, reversed the cables—the performance remained between approximately 1000MB/s and 2100MB/s. Fastest times, not surprisingly, coming from the reads.
How can this be? I mean, yesterday my speeds were in the sickly range of 50MB/s. What changed? I wondered if screen-sharing had anything to do with it. It was the only thing about this machine that was different than it had been the day before. But when the technician disconnected, the performance of the external SSD didn't wither.
The only thing I can guess, and it's a weak guess, is that the ports in this new MBP were initially so extremely tight, they were impeding a proper connection.
Both of the support techs confirmed that Apple is aware of the external-SSD throughput issue. The implication was that it's likely related to the OS and is not a hardware issue. (Though in my instance at least, it may have been hardware—those ports.
I remain more than a bit baffled. Am hoping the gremlin doesn't reappear.