Hi Dana,
in the new version of sapUI5 behaviour of dialog buttons has changed. Before, when aggregation had only one button, it was stretched for 100% width of dialog, if two, both buttons had 50% of width. Right now buttons are displayed to the right by default. Here is a css code that should fix it and make dialog buttons look like in previous versions of sapUI5.
.sapMDialog > footer { display: flex;
}
.sapMDialog > footer > .sapMTBSpacer { display: none;
}
.sapMDialog > footer > button { width: auto !important; margin-right: 0px !important; margin-left: 0px !important; flex-grow: 1;
}