Hello BeniJet,
We know it is important that your contacts are always available when you need them. Let's look deeper and try to find why they are disappearing.
Power cycle the iPhone using these instructions: Restart your iPhone
While simple, this can refresh the iOS, apps and connections.
If the contacts continue to disappear, on the iPhone, tap on Settings > Contacts > Accounts. Tap on the Outlook account. Make sure that Contacts is toggled on.
Do you have other Apple devices that share these contacts? Do they disappear from there also?
We hope this helps.
Have a great day!