Hi @Jay ,
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What's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.
Does the issue occur when all messages with attachments are forwarded? If the issue occurs when forwarding messages with specific attachments or forwarding specific emails, do these specific attachments or emails have any similarities?
Based on my tests on Outlook 365(version 2109(Build 14430.20270 Click-to-Run)), the attachments were not renamed when I forwarded the emails with attachments, which didn’t reproduce your issue. Considering the impact of some add ins, It is suggested that you try to start outlook in safe mode(win+R>type: outlook /safe) to check whether it works normally.
If this is still the issue in safe mode, it is suggested that you could try to create a new profile(control panel>mail>show profile>add) and add the account to check if there are any differences.
If the above is invalid, considering the Outlook client itself may be damaged. It is recommended that you try to fix the Office application to check if there're any differences.
Hope the above!
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