Thank you for posting your question in the Microsoft Q&A forum.
Based on the information you have provided, looks like you have problem with printing in New Outlook.
Make sure you disable "Let Windows manage my default printer" in Settings > Device > Printers
Please try disable this feature in the system printer, it has an option to prevent Outlook changing the preferences.
Additionally, go to Control Panel > Program > Turn Windows features on or off
Make sure this is on.
If the issue persists, please try to clear Outlook cache.
Close Outlook app > Press Windows + R and type in this command:
%localappdata%\Microsoft\Outlook
Select RoamCache folder and clear all files in it.
I hope this information helps. Please let me know if it works for you.
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