Breaking change somewhere in recent Raspberry Pi OS update

Yesterday, did an upgrade on some Raspberry Pi 3B+ that use this library to drive a newpixel ring as part of an interface application. After the upgrade, the rings stopped functioning - even the example scripts provided in the repo fail to display properly. On the off-chance that something went wrong with the upgrade, I tried a fresh image on a fresh SD card and installed only this library but got the same results.
Of the 24 neopixels in the ring. only the first 4 or 5 do anything and they tend to be flickering random colours, mainly greens, blue and sometimes white.
It's not a hardware fault as if I transfer the ring to another device that hasn't been upgraded it works perfectly.

Start by reproducing the failure with the repository's example scripts on a Raspberry Pi 3B+ after the Raspberry Pi OS upgrade, then compare it with the unupgraded device that works. Investigate the upgrade or fresh-install differences visible during reproduction; done means all 24 NeoPixels display consistently as they did before.

Written by the indexing model from the issue text.