Current tooltips implementation doesn't work with BS5.

BS5 has tooltips disabled by default. These need to be enabled e.g. on page load via JS.
this can be done by selecting e.g. all elements on page that have class (hasTooltip): the class Kunena currently adds to elements.
Issue here is that this is not enough.
The element that has the class hasTooltip should also have the attribute title, this contains the content of the tooltip.

What I see now (e.g. on the avatar in a message) is e.g.

<span class="kwho-user hasTooltip">
    <span class="rounded" title="Groenewoud's Profielfoto">

This will not work as the element that has the tooltip class doesn't have a title

I also see:

<span class="heading btn btn-outline-primary border btn-xs hasTooltip" data-original-title="Klik om meer profielitems van deze gebruiker te zien">

where data-original-tooltip is used instead of title

So implementing tooltips should be rethought / refactored as the current implementation is a mix of things that will not work in a consistent way.

btw: the current tooltips you are seeing are browser tooltips that just show the title attribute when present: they show always regardsless of settings as the show not show toggle here is does it have a title attribute or not.