Is keyboard focus synchronization broke in Mac mail?

I've noticed odd behavior in the Mac Mail app and wanted people's opinion on whether this is a bug that I should report. Note that in VO Utility, I have "Synchronize keyboard focus and VoiceOver cursor" selected. This is the default.

To reproduce:
1. Open a message from your inbox.
2. Your text cursor is on the first line, of course. Verify this with down arrow, then up arrow.
3. Press VO+F and search for text toward the bottom of the email, such as the sender's name, which is typically in the signature. You will hear VoiceOver announce the full line that it reads.
4. Press up arrow or down arrow.

Expected behavior: Because keyboard focus is synchronized to VoiceOver, step three should cause keyboard focus to jump to the target of the VO+F search. Up and down arrow should move relative to that line, and VoiceOver should read the line prior to, or just after, the line it announced in step 3.

Observed behavior: After step 3, keyboard focus doesn't move and remains at the top of the email message. Up arrow does nothing, and down arrow reads the second line of the message.

This is especially problematic if the message in question is a mailing list digest and you are trying to move to one specific message within the digest body.

I'm on 15.2. I'm unsure when I started seeing this issue, but it's been around for a while.