iCloud shared calendars won't stay in view only I'm trying to share a calendar with my partner, and she is attempting to do likewise with me.

The first issue is that when sharing a calendar you do not get to select a permissions level, which has to be amended after it has been shared from the default or read/write to view only.  There's no fix for this that I know of, it's just annoying and worth a feature request.

The second issue which is more annoying is that the permissions level for either calendar is reverting back to read/write no matter how many times I ask it to use view only.

I've attempted to change from iPhone/iMac and Web, and the same issue each time.

I've removed sharing access full stop and then re-shared the calendars, no luck.

I do have a shared calendar with someone else that has stayed in view only, but that was shared ages ago so I don't know what I might have done differently then.

Any clues what to do to fix this?
The only way I managed to change the permissions was via icloud.com. OS/X and iOS did nothing.