Bevel Modifyer increases the size of one face

Hi there

I wanted the use the Bevel modifyer just to round the edges of this wall a little.
The modifyer works fine so far, just increases the size of one face and I don’t know why.
I applied all Transforms (Ctrl+A) and I just cannot understand, why one of these faces reacts differently. The face in the background should look like the one in the foreground actually.
an image of the increased face with the bevel modifyer used

is attached
Thanks in advance!

Are your normals pointing consistently outside?

Thank you!!!