Greetings, philixer.
I see that, when you ask Siri to set an appointment for you, Siri confirms it's created. But, when you ask what appointments you have, it hasn't been added, and I'm happy to help.
Restart your iPhone, iPad, or iPod touch -- Restart your iPhone. Once it's returned to the Home screen, see if Siri adds the event.
How to back up your iPhone, iPad, and iPod touch -- If restarting your iPhone didn't resolve the issue, please be sure to have a fresh backup to keep your personal information safe. In order to back up your Health data, the backup needs to be encrypted. iCloud does this automatically, but iTunes doesn't. If you'd like to use iTunes to back up your iPhone, refer to this article: About encrypted backups in iTunes
If you can't update or restore your iPhone, iPad, or iPod touch -- When an application isn't working as expected, a good step is to delete and re-add the app. Since you can't delete the Siri or the Calendar application on your iPhone, I'd like for you to reinstall the iOS. The section titled "Put your device in recovery mode and set it up again" will walk you through all the steps to reinstall the iOS software on your device. See if Siri adds the event correctly.
Let me know how that goes, and take care!