This project doesn’t utilise alternating time signatures, so it’s unlikely to be related to the thread that PjotrB linked to.
Did you type these bars into a new Dorico project from scratch, or import them as MusicXML from another program?
Incidentally, this happened to me again just yesterday in another piece. I just wanted to make a 2/4 bar into a 3/4 bar. I tried to just change the time sig, but it didn’t work smoothly. I had to delete the entire bar first, then add it back in as a 3/4 bar. Even then, it still wanted to be a default 4/4 bar followed by a 1/4 mini-bar. After I deleted the mini-bar it was fine.
You really should find that provided you have Insert mode switched on when you create the new time signature, the bar in which the time signature is added will correctly add up to the length of the new time signature.