gerardnico thanks for that plugin. I restored the Wiki start page to its previous state (removing the added newlines) and this is the JSON dump of that page using the plugin: https://paste.debian.net/1200082/
Prior to posting here, I had already gone through the exercises of invalidating cache, and disabling plugins.
However, so far I have disabled plugins only one by one, checking whether anything would change after disabling that plugin, and re-enabling the plugin if I did not see any changes in Wiki behavior.
But the thing is, the Wiki has been running for years without issue, and the bad page rendering did not start after adding a plugin. The rendering went bad somewhere after doing a fairly large OS update and adding some virtual hosts to the web service. There was a bug in Apache 2.4.37 which affected two of my web sites and during tests a lot of httpd test upgrades happened but nothing relevant changed in the httpd configuration (apart from adding those test VHosts to help resolve the Apache regression).
I don't open my Wiki everyday, so I discovered the issue only when someone pointed out the weird page layout.
I feared it was caused by the recent OS package upgrades. One of the first things I did was reverting php and http to their previous versions, which did not make a difference, and then upgrading Dokuwiki to the latest release. Also updated all the plugins to their latest releases (and had to fix several of them after Hogfather turned PHP warnings into errors).
I have now a test version of Dokuwiki running on the same package set (same versions also) of Slackware Linux, and the instance here on the laptop is fine. On the other hand, the pristine test instance on my online host (running docs.slackware.com and orwell.alienbase.nl) does have the rendering issue. It's a VHost using the same Apache base configuration as docs.slackware.com.
I would therefore rule out a software version issue but rather think it is a software configuration issue.
If you cannot pinpoint a possible cause by looking at that dump data, I'll have to find that needle in the haystack... I have daily backups of /etc/ and should hopefully find something relevant if the dump doesn't reveal anything,