I'm an academic mathematician. Your pointing out the name of the journal is an interesting point.
Pop quiz: out of the journals Aequationes mathematicae, Indagationes mathematicae, Inventiones mathematicae, Quaestiones mathematicae, three are mediocre and one is absolutely top-notch. Without googling can you guess which?
MDPI is a publisher that was started recently, has apparently tried to grow very fast, and engages in questionable practices. I peer-reviewed for one of their journals once and had a very negative experience. Never. Again.
They've also started journals with good sounding names. For example there was no mathematics journal with the title "Mathematics" so they started one.
I've instructed my mail client that *@mdpi.com is to go straight to spam. I got invited to be on the editorial board of one of their journals, and I felt no guilt about not responding. Not too surprised to see them publish crackpottery.