TYPO3 Core

Same here.

This:

{f:uri.action(action: 'show', controller: 'Event', pageUid: settings.detailPid, arguments: '{event: eventItem}')}

Creates a relative URL like

/?tx_myext_events%5Baction%5D=show&tx_myext_events%5Bcontroller%5D=Event&tx_myext_events%5Bevent%5D=10&cHash=f3702d971c85744fa4ce8416a43c4665

It is missing the whole URL part.
My workaround is a f:link.typolink with the f:uri.action as parameter.

{f:uri.typolink(parameter: '{f:uri.action(action: \'show\', controller: \'Event\', pageUid: settings.detailPid, arguments: \'{event: eventItem}\')}')}

Does is it work with absolute:'true' for you?

Works for me. Seems that there is no Site configured in Site Management?

Creating a site doesn't fix it by default because the content of the base matters for the functions uri.typolink and uri.action

Possible fix?
base: 'https://%env(HTTP_HOST)%/' in the site managemant module?