Iphone calendar alerts disabled

i’ve always had alerts from my calendar. But recently it seems to have stopped.

When I check the alert setting on any given invite, it is struck through. And when I click into the alert in the invite, it offers me to set a time like normal but at the top it reads “Alerts for this calendar are disabled. To allow this alert to fire, enable ‘event alerts’ in the calendar settings for ‘Calendar’.

So I follow these instructions. I go into settings > calendar but there is no ‘event alerts’ within this menu for me to enable.

I also tried going into settings > notifications > calendar. But everything is fully enabled - nothing disabled. I also check in settings > notifications > calendar > customised notifications and I can see the ‘upcoming events’ is turned on (every other option is also turned on).

The only thing I can think was a spam invite I got about 2months ago and I struggled to delete it but eventually did. It sort of lines up with when I stopped getting alerts.

appreciate any help you can provide