As we discussed, this is not a bug but a design desicion to be finalized.
The current idea is to use the YAML fronmatter for dev\plugins metadata.
Users should not really use the YAML metadata directly. If users want to use a frontmatter, they can do so outside the YAML block, i.e.
---
YAML metadata
---
your frontmatter
---
or
---
YAML metadata
---
# your frontmatter
bla bla bla
# main text