LeftieBiker - thanks! I know that single bad cells & other defects are covered, just don't know how easy it is to get warranty service with occasional & not super obvious symptoms. No obvious bad cells in leafspy, no spectacular failures.
goldbrick - It seems to usually perform as expected on flatter roads. Reporting 4-4.1 mi/kwh average, which seems plausible and fits acceptably with the models. To confuse matters further I looked at logs for a shorter and steeper climb (12ish miles, 3600', 35-40mph) and car overperformed. Within model error for sure but still. Batt was 100% on level 1 just before & not too hot (85-93). Can those things really make the difference? I thought hot batteries notionally had lower internal R even as they were dying faster, but maybe it effectively drains the already weaker cells more? Or internal R isn't a great model for lithium cells.
Honestly thanks for the replies. It's the mystery of it that bugs me (and, of course, the prospect of stranding in the desert at midnight). If just have to be much more careful about heat & climbing combined, or top balancing under these circumstances, that's not so scary. Irritating but we can deal.