Does not sync with Fossify calendar

[cccslater]

Hi,
I'm using Lineage 14.1 (android 7.1), ICSx⁵ 2.2.4, fossify calendar 1.0.3 (both apps via f-droid).
I have successfully subscribed to a webcal, but it doesn't show up in fossify calendar.
This is a very very old phone so I'm not expecting loads of support -- I just wondered if ICSx⁵ downloads the ics file somewhere each time it syncs? Then I could assumedly import that ics into fossify calendar. However I doubt this would automatically re-sync.
Thanks!

[cccslater]

Tried a different calendar (etar). works.