MTProto is not end-to-end. MTProto is their obfuscated client-server transport encryption.
What the commenter above is referring to is Telegram defaulting to saving your messages on the server in plaintext. You can use a "secret chat" which enables end-to-end encryption, but that is separate from MTProto.
Your sentiment is correct though. Messages should not be visible in plaintext to the server.
I first got this realization when I started using grey scale mode for my phone at night. A "good to bad" scale in an app became unintelligible. Since then I try to consider colorblindness if I design stuff myself. It's fantastic if color scales carry meaning in both their colour but also the same meaning in their lightness, so everyone can understand them the same.
They likely use adblocks themselves.