Buttons on webpages can be HTML coded, or in javascript as a direct frontend.
E.g. if I keyboard tab to a "reply"-button on this webpage - here "reply" is a button that is html implemented - this would not trigger this behaviour. After pressing (enter) the button, the next keyboard tab would move to the next item on the webpage.
However when I keyboard press (enter) a javascript frontend button, and then press tab again, I do not select the next item on the webpage, but the first item on the top of the webpage instead.
Since the webpage is work-related I cannot share it, but this is a definitive change in safari since Sonoma, which made me switch to Chrome, which however is not my preferred browser for these tasks.