Missing information when folder as source

Hello everyone

I created a new dataset yesterday and as source i used a folder. In this folder are csv files from every day back to 01/01/19.

Today i saw that i have [null] entries in a column, but if i use one of these files with null entries as single csv source i don't have this problem.

If there just would be 10 or 20 it would not be a problem but it affects more than 65% of 1.23M values.

I checked a Dataset which i created almoste a year ago, sam path and folder as source and i don't have this problem.

Tried it on two different machines with the same result.

I have the same pattern when i use power querry editor in excel.

Update: If i export the data again from the source system it shows the missing value, but this is not an option because there are 750 export files.

Perhaps someone has an idea why or how this is happen.

Regards

Robin