Dear NJLR,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
I just tested from my side, and it seems that I cannot meet the same behaviour as like you described above.
In Word, Letters can seem to disappear in Word if you're typing in overtype mode or have a borderless text box covering text. Words can also disappear if you have field codesin a document.
If possible, could you please check below setting and verify the result with me.
1.Open Word, click File > Options>Click Advanced>Under Editing options, clear both the Use the Insert key to control overtype mode and the Use overtype mode check boxes.
2.Go into paragraph formatting (drop-down arrow) and try to turn off "Widow/Orphan control" for the Normal style.

Meanwhile, may I know does issue happened do specific Word document? If the issue only happens to a specific file, the file may be corrupted. In this scenario, please copy/paste content to a new created Word file, then to check the outcomes.
If the issue happens to all Word files including a new created one, the issue is more related to the Office app itself or your computer environment.
As the problem can be caused by different reason, I will appreciate you can try the normal troubleshooting steps 1 by 1 below and tell us the result.
- Make sure you’ve updated Office to the latest version that some issues can be resolved with such method.
If this situation disappeared in Safe Mode, then this issue could be caused by the incompatible add-ins, you can disable add-ins by this way: Open Word>add-ins>Disable add-ins. For reference: disable application add-ins and COM add-ins
- To check if this problem might be caused by the incompatible add, if possible, could you please try open your Word insafe mode which allow you to safely use it when it has encountered certain problems
- If still can’t work, please try torepair an Office application to fix the problem.
In the meantime, to confirm whether this is a bug in a specific version, please provide us the version information. To check version: Open Word>File>Account>Product Information and capture a full screenshot and share here and we will be tested on that version and verify the result with you.
Also, please confirm, whether other applications also get the same problem? Such as PowerPoint or Excel and etc. It will help for us to narrow down your problem.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad