Threaded view doesn't work as expected

The "Threaded view" option doesn't work, or at least not in the way I expect it to. I've attached two screenshots of the same conversation, once of thunderbird and once of proton mail. Proton does chain the email thread properly, but in thunderbird it's completely detached. Email conversations get really painful without proper email chaning/threading, especially if a replyee's client doesn't attach a quote ("> ...") at the end.