Numbers deleting data?!?!

I thought I would see if anyone else was having this problem. I am in numbers with a spreadsheet that has several tabs (my classes), I enter in some data into a column (test scores), and then move to a new tab to do the same thing. Everything seems fine, until I go back to the first tab because I found a score I didn't enter and the previous ones I entered are now gone?!?

It doesn't do it all the time only sometimes, and only with some pieces of data. One column can have things save just fine and others disappear.

I have tried exiting out of the app, restarting the iPad and it doesn't really seem to have an effect. Sometimes it works while other times it doesn't.

This is problematic for me using it as a gradebook and losing the grades of my students. Has anyone experienced this and been able to reliably solve it?

Thanks in advance