Organizing and accessing contact information in Outlook.com
Hi,
Typically the contact needs to be deleted and re-added to fix this issue.
If it does not solve, I wish I had a better answer for you, but Microsoft no longer provides an ability to delete individual entries in the Autocomplete list, which was the next ideal solution.
https://feedbackportal.microsoft.com/feedback/idea/97490d3b-7d45-f111-9a91-000d3a05c4f5
Community member HornBlower has provided some complex instructions which may help you find if Autocomplete is the culprit:
I hope this information helps. If you have any more questions, don't hesitate to reach out. Wishing you good luck!