The ICS sources are available here : https://www.data.gouv.fr/fr/datasets/le-calendrier-scolaire/ and the links I tried to use are https://fr.ftp.opendatasoft.com/openscol/fr-en-calendrier-scolaire/Zone-B.ics
This feed contains invalid data:DTSTAMP:20211001T010008Z DTSTART;VALUE=DATE:20220707 DTEND: ← no value, invalid! SUMMARY:Vacances d'Été LOCATION:Zone B
Also, the events don’t have an UID, so all events must be re-synced at every sync. I recommend to contact the vendor to fix these problems:invalid DTEND entry missing UIDs of events
You can also check with https://icalendar.org/validator.html to see the problems.