Jack Kamm <jackkamm@gmail.com> writes:

>> Does the attached patch fix the problem for you?
>
> I can reproduce the bug by calling `org-icalendar-export-current-agenda'
> in my agenda buffer. It seems to mainly happen when subsequent agenda
> entries originate from different files.
>
> Ihor's patch fixes the problem on my end.

Thanks for testing!
Applied, onto main.
https://git.savannah.gnu.org/cgit/emacs/org-mode.git/commit/?id=14bde5aba
Fixed.