Hello AntoineD11!
Thank you for bringing this to our attention and we appreciate your time posting your Office concern here in Microsoft Community Forum.
We understand that this situation is difficult and it might be affecting your business let’s work on this together as team to get to the bottom of this.
- Have you tried using a different mouse?
- Can you please try to select text in Word online and see if you will be experiencing the same problem. Click here: Word
- Are you only experiencing this issue when you're on a DEX mode? how about on a normal Windows computer? is it working fine?
- What is your Word app version? ( ex. Microsoft 365 for Business or Microsoft 365 Family/Personal) Please go to File>Account, capture a screenshot of Product Information, as shown below.

Now in order for us to better assist you on this matter I need to ask for a screen recording of the issue. We want to see the actual issue happening on your end in that way we can also replicate the issue here on our end to provide you answer or resolution and also to make sure that we will not missing anything. Please make sure all the things you've performed will be included on the video.
Record your screen using Microsoft PowerPoint click here: -- Record your screen in PowerPoint .
Please upload your PowerPoint file to OneDrive click on share then click copy link and provide it here when you reply.
Here's how you upload files to OneDrive. -- Upload and save files and folders to OneDrive****
To share or copy the link of your file, please click here. -- Share files and folders in OneDrive (personal)****
We look forward to your response.
Sincerely,Juhn Jac,Consumer M365 Forum Moderator