Hi there! Just to confirm, is this happening to all emails being sent to your account or is it only from this specific sender?
If you are using Outlook on the web, I suggest to try going to Settings > Mail > Rules and look for any rule that could be moving, deleting, or otherwise filtering messages from this sender. Also check Deleted Items, Archive, Other, and any custom folders you may have and also the "Other" tab of the Outlook.
Also, try to temporarily remove the sender from Safe Senders, save the settings, and then add the address back again. Make sure the exact email address is listed rather than just the sender's display name. Safe Senders are normally excluded from Junk filtering, so if the messages aren't even reaching Junk, something else may be happening to their end instead.
If possible, check whether they receive a bounce/non-delivery message. That can help determine whether the message is being rejected before it reaches your mailbox. For reference, you can also send the Microsoft support page to this sender so they can follow the guide from the Microsoft support page to fix any possible issue on their end:
https://support.microsoft.com/pl-pl/outlook/sender-support-in-outlook-com