First: 5.6 is very old too (and already end of life), you should consider going to 7.2 or 7.3
Concering the show_recurrent_events_once: I’m guessing you’re hitting the limit of the sql statemnt I can provide. First, from the changelog:
* Fix a show_recurrent_events_once sql bug (in fact not a bug, but indeterministic result) due to newer mysql/mariadb versions
This fix resulted in that I needed to get all recurrent events as if standalone ones, and doing the “limit” later on (which is what you are probably bumping into: increasing the limit will solve your problem). Another option you could consider is using period-based showing of events (like the next 2 weeks or so), which doesn’t bump into this limit-thing.