slight correction: my pipeline is still running, but otherwise is as described above at this point. The downstream steps that were complete are now completely missing in the visualization. The parallel-sequential step updated to show all complete (they were all complete ~ 40 minutes earlier, in reality). When the UI update occurred for that, that's when the downstream sequential steps that had previously run and completed (and were shown) disappeared. The pipeline is now on a step further downstream and it's running, though it's not shown anywhere. I suspect that when the entire pipeline is complete, it will all reappear, as I think I saw that occur before.
update: as suspected. The entire pipeline shows upon completion, including the steps that had shown before that went away mid pipeline as described above.