@outline.games domain definitely has authentication issue as SPF/DKIM/DMARC are all missing. Microsoft can outright reject their incoming traffic without bounce email. You said you still received their emails, it can be just a matter of time before their emails will stop coming in altogether when it reaches the volume sender threshold.

@itv.com on the other hand, seems to have all three records set up.
However, this is their SPF record. If Microsoft 365 is not included there, then yes, Teams invitations sent from that domain can be rejected or marked suspicious. Their sof record should have included "include:spf.protection.outlook.com"

I honestly don't think this is something you can fix from your end. Their IT/email admins need to fix on their end.