Everyone: thank you so much for taking the time to reply.
Matt, though I didn't mention it specifically, I did actually agitate for about a minute at the start, as I always do. Here, from the last time I did this, are more details on the protocol, as well as the result I was expecting:
Using the metal tank did indeed mean I was using less of the chemical. But at 1:25, that means I went from an absolute amount of 16 ml to 12 ml, so I wouldn't think that could be the difference with Rodinal, which works in stand protocols with 5 ml developer in 500 ml total volume.
Regarding the scan, I forgot to mention that I did indeed examine the negatives very carefully with a loupe, and I can easily see the "posterization" edges, so it's not the scan.
Based on all the information I'm getting here, I'm leaning toward this: my fixer was marginal, I didn't fix long enough or agitate well, and in combination with the decreased access space in the metal reels, that killed it for me.
More experiments on the near horizon should support or refute this.