I agree - “sometimes” doesn’t sound good, it even isn’t good! The issue is reproducible, unfortunately not at will, but it has hit me several times during the last days. I’ve already tried to pinpoint the issue by using different WLANs, VPN, different time of day,… but this doesn’t seem to have any influence. It seems to boil down to somewhere between the server providing the .ics and ICSDroid. Even getting different calendars from the same server at (almost) the same time yields in different behaviour (see screenshot)
Let me know if I can provide further details (log files?) to help to solve this issue.