You asked why open source would matter. In this case it would matter because it is visible if the app is abusing an algorithm to keep you on the app rather than actually try to find a match you might like.

So for one, it's much less likely to be happening because anybody with the know-how can see it and tell people. If an app got a bit larger of a userbase and then tried to change it to be shitty, you'll know and can stop using it.

It's also less likely because typically these open source apps the person is developing for free (maybe with donations to help support it) so there isn't that incentive to do anything scummy.

Anyone else can fork the application and host it themselves, so if the original developer starts making changes you disagree with you and a few other contributors could come together and take them out (like CoMaps for an example).