How to stop or prevent from Microsoft writing weird.

Are you saving the files locally, or are you saving them to a cloud-based service such as Google Drive or One Drive?

Are you possibly off somewhere deep into Microsoft antiquity and arcana, the Microsoft FAT-format file system, and its 8.3 file-naming limit, and the tilde-containing short names?

This stuff: https://en.wikipedia.org/wiki/8.3_filename

File menu -> Save As. That will allow you to select the location. Or save the attachment to your preferred location first and then open it for editing. Then when you save the changes they should be saved to the same document in the same location.

Regards.

Mail keeps a cache of attachments, separate from those attachments explicitly downloaded and saved.

Office may well be doing who-knows-what with its files here too, though I am less than familiar with that.

It would help if you shared with us what Apple device you that shows this behavior and its operating system version. Possible responses may be different for a Mac computer than an iPhone or iPad.