There is no matchmaking outside of ranked. This goes for Frontlines and 8v8 Feast alike. Parties are matched randomly with no consideration for party composition - sometimes there will be a party with 4 healers when the two others have none, showing that there is literally no effort made to split or match them evenly. That also means the queue times are the same for everyone regardless of the class they queue up as, so that's not what explains switching from healer to DPS. Usually, it's when a healer isn't comfortable solo-healing and makes a statement that they won't do it unless someone else goes healer with them, but of course there may be other explanations, like if they never meant to heal and simply queued on the wrong class by mistake.

I think there used to be matchmaking in those modes, but SE removed it because queue times were getting completely out of hand (mostly due to lack of healers), so they allowed games to happen with whatever people queued up. However I wasn't playing at that time, so I may be wrong there.