Threading not picked up correctly (possibly)

Under some conditions, messages that ought to be threaded are not.

First, see the attached screen captures. Each shows one message being selected in the main outline. Threading is turned on, but they are not shown as part of the same thread in the main outline. HOWEVER, they are shown as part of the same thread in the "Thread" view on the lower left corner. My intuition is that the thread view should behave like threading in the main outline. Are there different criteria used?

Second, here are some details. In fact the two messages are addressed to two different addresses: one is to "surname, name (username)" username@MAIL.HOST.EDU; the other is to "name surname" name.surname@host.edu". Both of these resolve to the same mailbox (I don't know the terminology: the first is old, the second was added by my institution later, but they mean the same thing).

In the IMAP Account Setting:
Full Name: name surname
Email Addresses: name.surname@host.edu
Address Pattern: username@MAIL.HOST.EDU|username@mail.host.edu|USERNAME@.MAIL.HOST.EDU|USERNAME@mail.host.edu

(I tried to add the permutations for capitalization in case this was an issue, I suspect they are completely irrelevant).

Third, full disclosure:
I use MailMate on MacOS, but Spark on iOS. This situation arrises when I reply to some of the messages in the thread from iOS. Perhaps Spark is messing things up, but not for the "Thread" view.

Thanks!! I can send messages or the layout settings, if those are handy.