Hi @Kurt Stichelmans,
Thank you for posting to Microsoft Community.
Based on your description, I want to confirm with you if you mean the issue is that email not received by recipients in M365 but Exchange log saying it is delivered to M365.
If so, please use detailed mail flow to check where the email was delivered to.
Please check if there are any inbox rules in the mailbox might have moved or deleted the email and please check if the email was sent to quarantine.
Also, have you tried the OWA to see if it works.
Hope it helps, and if there is anything else you need help, please let me know.