Hey, first of all, thanks for for sharing and I do appreciate both Slint existing and you being able to do software that's usable by both businesses and, to some extend, open-source projects! (The latter depends on whether you consider contributing to the underlying libraries as a requirement for development, and if you're then fine with contributing with these MIT/non-MIT specifics.)
When you contribute to any MIT license project you are in the same situation
I would disagree here. If you're speaking about any MIT project, then many of them would be simply MIT. You contribute like MIT and you can use the code as MIT. Slint is not licensed as MIT-0 though. It's licensed as written here: https://github.com/slint-ui/slint?tab=readme-ov-file#license, and only your contributions are taken as MIT. This does set Slint apart.
It's a fair model though, if the developers are sufficiently aware of the deal. And it's a very sensible business model. I have nothing against it, and I only wish to make the exact deal more explicit. As you see around, I don't think it's 100% clear from the first glance.