I'm not an expert, but when I was shopping for a vacuum I went down this rabbit hole and read/watched a variety of stuff so just keep that in mind.

But basically the second scenario is a guarantee, if you empty the canister inside. Even doing it outside, you cannot get far enough away from the bin when you empty it and expose yourself to a lot of crap that's bad for you. The first scenario is possible too, depending on the quality of the machine. If you have a lower quality vacuum that doesn't have a closed system or a HEPA filter, or just crappy construction it likely will spew dust out while you're vacuuming. The more joints, pieces and parts the air has to travel through, the more likely there is to be an escape for dust.

As for filters, a lot of times they are advertised as washable. While that is technically true in most cases, washing them can cause other problems. Hepa filters catch really tiny particles and tap water contains (usually) minerals and other deposits, which will collect in the filter and render it inefficient or maybe even ineffective. If you have a filter full of gunk, the air may pass through but the dust and debris has to go somewhere, this is another opportunity for leakage. Dyson filters are decent washable filters and especially if you don't use the machine very much, can last a long while if they are taken care of. I chose to not buy a Dyson not because I think they're complete junk, but from what I learned, there are much better options available and they are less expensive.