There is a change to the rendering ChoiceDialog, if there are more than 2 buttons, from v11 to v12. The examples below are from evaluating the input cell of the 2nd of the two Basic Examples in the documentation
V11.3
![enter image description here](https://community.wolfram.com//c/portal/getImageAttachment?filename=v11.3.png&userId=93385)
V12
![enter image description here](https://community.wolfram.com//c/portal/getImageAttachment?filename=v12.png&userId=93385)
Note that v12 has an, uncalled for, difference in the width of the buttons.