Round corner setting changes column behavior?

Hello, community

I have acknowledged a strange behavior that appears when rounding the corner of column 1 (of 3) within a row. Each column has a width of 33,33% – however the first column will behave in the frontend like it must be 100%, Column 2 & 3 will adjust and jump underneath.

Has anyone ever had this before, it seems like a bug, I really do want to round the top left corner of the first column (as I also round the top right of column 3, column 3 behaves like it should though)

I´d be very glad for some help (I am just a designer, not a developer) :woman_singer: