open_basedir was intentionally not enabled in any of my php.ini/php-fpm.ini files on this TEST server. However, it turns out that open_basedir is enabled by default in the Archlinux package in nextcloud.conf. It is located well up above and away from where you are instructed to add or uncomment the igbinary and redis extensions so not something you will be drawn to. It was not something I was even looking for as I had made the decision not to enable open_basedir on this test box that is not accessible from the outside world.
After grepping around through all the various config files (made a bit more complex given how Arch isolates Nextcloud per the webapp std), it was the stray grep of nextcloud.conf dumping it without comments that disclosed open_basedir being set as part of the webapp isolation.
BINGO! Once that was discovered, simply adding /run/redis (the runtime directory on Arch) to the open_basedir path allowed full caching and the MySQL transactional locking to work like clockwork.
So if faced with a similar issue on a distro that isn’t given as example in the Nextcloud documentation, double check ALL config files for open_basedir.