Notes randomly deleting

Goodbye

Please include as much of the following information as you know:

Version/revision of MuseScore you are using. Check Help → About... (Mac: MuseScore → About MuseScore...).
Operating system being used (e.g. Windows 10, macOS 10.15 or Ubuntu 20.04)

Attach the score that shows the problem, at least a minimal example — use the "File attachments" option at the bottom of the page, just above the Save and Preview buttons when you're typing your post.
Describe the precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.).

If you are not able to reproduce the problem with the steps, it is probably not worth reporting it as the developers will not be able to reproduce (and solve) it either. Remember that the goal of a bug report is not only to show the problem, but to allow others to reproduce it easily.

Also, see:
https://musescore.org/en/node/289899

Sorry to hear you having trouble! I've never heard of this happening before, so in order for us to investigate and hopefully fix whatever the problem is, we would need your help to reproduce the problem. Since you say it happens every time, it should be a simple matter to show us how to reproduce the problem. just attach your score, tell us what notes we need to add and where and how you are adding them. Also any other steps that would be necessary in order for us to reproduce the problem 9like maybe if it only happens if you also transpose the score after adding the note,s or whatever).