Issue confirmed, applies to invitation and local event.

Create event on thursday, that recurs weekly on friday (but not on thursday). The recurrence ends after three occurrences. (Note this is only possible in roundcube, kontact always implicitly includes thursday in the recurrence).

It is expected that we have a total of 4 occurrences (initially on thursday, then three times on friday), but kontact doesn't show the initial occurrence on thursday and starts directly with the first occurrence on friday.

Actually this seems to be a misinterpretation of the RFC https://tools.ietf.org/html/rfc5545#section-3.8.5.3

The "DTSTART" property defines
the first instance in the recurrence set.  The "DTSTART" property
value SHOULD be synchronized with the recurrence rule, if
specified.  The recurrence set generated with a "DTSTART" property
value not synchronized with the recurrence rule is undefined.

So the created event is per definition undefined.

Also, as per https://tools.ietf.org/html/rfc5545#section-3.3.10. (the table)
We see that BYDAY is expanding to the Weekly rule, which would at best mean that thursday would have to be included every week.
But that is irrelevant since the event is undefined in the first place.

Kontact correctly implicitly includes thursday in the BYDAY rule, but roundcube doesn't.