I tested on my laptop running the same set of packages as docs.slackware.com, but it has a completely default httpd configuration and no fiddling with any php configs. I don't use the laptop as a server, it runs KDE Plasma5 FYI.
I did the exact same as I documented above for orwell.alienbase.nl (which is just another VHost next to docs.slackare.com on the same hardware). On my laptop, I see zero issues, Dokuwiki looks fine, page renders as expected.
I compared my backups of /etc/php.ini and /etc/httpd from May 8th (when all was still fine) and June 4th (today) and the difference was that I added two new VHosts and there was a php.ini change of the value for 'memory_limit' (which went from the default 128M to 512M because Nextcloud recommended that).
Reverting to the May 8th configuration did not make a difference either to Dokuwiki's page rendering.
What did make a difference was by adding newlines to the Wiki source text. This was the original beginning of the start page, causing parts not to be rendered (see my initial report above):
~~NOTOC~~ ~~NOCACHE~~
====== Welcome to the Slackware Documentation Project ======
We aim to become your primary source for [[slackware:slackware|Slackware Linux]] [[start|documentation]] on the web.
And then I changed the text to this:
~~NOTOC~~
~~NOCACHE~~
====== Welcome to the Slackware Documentation Project ======
We aim to become your primary source for
[[slackware:slackware|Slackware Linux]]
[[start|documentation]] on the web.
With these additional newlines, which moves the syntax statements that fail to render to the beginning of a new line, the rendering is now correct, as you can see at the top of https://docs.slackware.com/
Does this help forming an idea of what causes this behaviour?
Thanks for the assist, Eric