is it bad convention to put loops with a single statement into 1 line

probably depends on case, but personally I never do this and we do not allow such code to pass code review. Also we just have very few while loops

You mean its bad practice and it should be surrounded by braces?

I prefer surround everything with braces

@Florian Wasn’t the original question about this?

list.forEach { println(it) }

vs

list.forEach { 
   println(it) 
}

Usually it doesn't called loop, at least I always think that loop is while and for. But maybe it's only my strange understanding and it look fine for me

No I meant without braces

Than I got it correctly, I would avoid such construction personally. Also most probably rewrite it as:

list.forEach(::print)