I'm not familiar with Swift, but in other languages, you should include a semicolon if a lack of one sometime means something different, as in JavaScript.
In JavaScript, it'll attempt to automatically join consecutive lines if you don't have a semicolon. It'll only add in a semicolon if implicitly joining the lines together would result in invalid syntax. This can result in really weird and hard to track bugs, so in JavaScript, you should always end lines in semicolons, even though they're supposedly optional in the language.
In Python, if you don't end a line in a semicolon, one will be added automatically. So in Python, semicolons are completely pointless and the best practice is to never use them. They're just meaningless clutter in Python.
So with Swift, you need to find out whether the presence of a semicolon ever changes how things are interpreted or not. If it never impacts it, then the best practice is to leave them off. If it sometimes impacts it, then the best practice is to always use them.
There is no language where mixing semicolons and non-semicolons is a good practice.