On the Web UI under DVR > Schedule, editing (reducing) the end padding of an in-progress recording is ignored and the recording continues as originally scheduled.
The Simpsons S34E02 was being recorded on FOX with 30 minutes of end padding (in case a sports event runs late) and 10 seconds of start padding. The airing in the guide is 8pm-8:30pm EDT, and the recording was scheduled by a Series Pass to record from 7:59:50pm until 9pm.
At 8:30pm, I briefly played the in-progress recording from the Web UI, and verified that the show had ended on-time. At 8:31pm, I opened the Schedule, clicked the corresponding Recording dropdown > Edit > Padding, and adjusted the end padding to just three minutes, with the expectation that recording would stop at 8:33pm to reflect the updated end padding for this airing; the recording duration changed from "1 hr 1 min" to "35 min" (presumably due to rounding/ceiling, or I accidentally selected five minutes). However, this change had no effect on the end time, since I still found it recording at 8:42pm, upon which I used the Recording dropdown > Stop Recording option in the Schedule, and now the recording has a red "Cancelled" badge.
Logs have been submitted as 2e2032a2-1b10-4069-8749-076b205f2ccf.