The Microsoft Lists iOS app won't open lists on the SharePoint root site. Lists from other sites within the tenant are unaffected e.g. any site located in /sites/
Raising this with Microsoft 365 Support, they have confirmed it is an issue with the iOS app.
The current workaround is to either use the SharePoint app for iOS or move the list to another site.
@Mark Kashman, I posted this on your Microsoft 365 blog post, but in hindsight this might be the more appropriate place. Hopefully this bug is fixed next time the app gets updated.
