Repeated events: changed event disappears

[HelmutL]

There are repeating events, say the 20th every three months.
If you change the date of one of these events (lets say the 3rd one), it is not shown any more. The preceding events of this chain are still ok, also the folwoing events (at least in most cases).

calendar 2.0.1
owncloud 18.0.1
PHP 7.3

Note:
The evnets still are presented ok (i.e. thewhole chain, including thechanged event) in clients using caldav / DavX