Thanks for the screenshot, Rolf and you were right all along!
Following up on my earlier reply: after reproducing this the way you described (adding a third all-day event to the same day), the "+N more" indicator appears in the built app on macOS but not on Windows or Linux. Digging into it, that small overflow label is laid out in a space that is too short for the text line height that Windows and Linux font rendering produces, and the text engine then omits the line entirely rather than shrinking it so it never displays on those platforms. In other words, the indicator has effectively never worked on Windows or Linux since the Calendar control shipped, which matches exactly what you reported.
The fix is tracked and will address both halves of your report: the all-day area will grow to show more all-day events instead of stopping at two, and the overflow indicator will be laid out so it displays reliably on every platform. You can follow progress at https://feedback.objo.dev/feature/1664. Apologies for the confusion in my first reply - your report was accurate.
