dspillett was exactly right: ps, e.g., truncated its output at $COLUMNS and there was no horizontal scroll.

As suggested above, it did this even when called from a script.

The fix was easy, set COLUMNS ridiculously large if DEC Unix, but it took days of WTF apparent UB before I realized how simple was what was happening. It just seemed haphazard: I'd reposition and resize a window so I could run the script in one while manually running the commands in another, get inconsistent results, rinse, repeat...

...and eventually realize the common element in each test was me, and the variations I was introducing were window size.

I cursed their engineers for trying to be "helpful" and keep things "pretty".