mu4e now incorrectly marks messages as part of a thread and duplicate

### What did you expect to happen?

mu4e should not indicate a message is in a thread, or a duplicate, when it's not.

### What actually happened?

Somewhere between 2731685095d1e6101b3215aa689426e1834ce00f and faceb8cef6f86524f4256915167863a1854aa76b, the Doom changes for mu4e caused messages that are part of no thread, have no threading headers, etc. to have this prepended in the Subject line:

└>=

I also have another machine still on 2731685095d1e6101b3215aa689426e1834ce00f and it doesn't exhibit this issue.

### Describe your attempts to resolve the issue

maildir-utils is 1.4.15-1 on both machines.

I long ago set these lines in my config, and they are still present:

```
(setq mu4e-headers-skip-duplicates nil)
(setq mu4e-headers-include-related nil)
```

### Steps to reproduce

Open mu4e and go to my inbox.

### System Information

https://pastebin.com/UQg2guiN