After iPhone IOS 18 Upgrade Calendar App not Synching

Since upgrading to IOS 18 on my iPhone, iPad and iMac (Sequoia 15.0) only the iPhone has new calendar entries from third parties (like my gym reservations from Lifetime Fitness). I tried everything suggested by Apple Support and nothing helped.

Finally I added Google Calendar as an internet account and was able to see the new entries made on my iPhone. Not sure why this happened after the upgrade.