Do you get any errors in the console? Breadcrumbs logs some nested warnings as well, so be sure to dig a bit

I don't see anything in the console at all from these actions

markdown.md:

---
BC-tag-note-tag: abc
BC-tag-note-field: down
---

test.md:

---
tags:
  - abc
aliases:
  - bumble
---

Does it work when you create the #tag in note body instead of yaml?

I am having this same issue, but I am on v4.2.35-beta.
OS: Arch Linux
Breadcrumbs Version: v4.2.35-beta
Obsidian Version: v1.8.4

I encounter this problem on a fresh vault with only BRAT and the breadcrumbs beta installed. It does seem to work when not on the beta, for me. It also works when the #tag is in the note body, rather than the frontmatter.

markdown.md:

---
BC-tag-note-tag: markdown
BC-tag-note-field: down
---

test.md:

---
tags:
  - markdown
---

This is the output in the dev console when the Debug Level is set to DEBUG and I restart Obsidian:

I have tried the following:

• Adding/removing the BC-tag-note-field property
• Reinstalling, testing in a fresh vault
• Trying the non-beta version (seems to work as expected)
• Putting the tag in the body (this does work)
• Resetting settings, tweaking various settings
• deleting and remaking notes
• trying various tags
• using the old BC-tag-note field instead