Context: Slacwkare64-15.0 fully up to date with added packages including a full featured MATE desktop and its dependencies.

If I press F1 or select Help->Summary from the menu of a MATE application, after about 30 seconds khelpcenter raises a window telling that there is no help for the application in concern, regardless of the desktop in use and the run level

But if I remove the package khelpcenter, a window with the expected help menu for this application is raised, as expected.

I will be glad for any clue, hint or advice how-to investigate this is issue.

I just tested this myself since I am a mate user and can confirm the issue. I haven't found a fix, bit if I find anything I will report it here.

i can confirm this as well

Possible explanation:
the package khelpcenter-21.12.1-x86_64-1.txz provides these two files:
/usr/share/applications/org.kde.Help.desktop
/usr/share/kservices5/khelpcenter.desktop
It seems that the latter somehow takes precedence over the former, additionally it doesn't honor the OnlyShowIn and NotShowIn settings.

Workaround: Then yelp is called whenever expected.