.markdown files not processed from custom extension alias (v3.0.0-alpha.11 and above)

Operating system

macOS Sonoma 14.5

Eleventy

v3.0.0-alpha.11+

Describe the bug

Using any version of v3.0.0-alpha after alpha.10 results in 80% of files no longer being written to disk:

• Copied 2046 files / Wrote 2647 files in 8.08 seconds (3.1ms each, v3.0.0-alpha.10)
• Copied 2046 files / Wrote 556 files in 2.59 seconds (4.7ms each, v3.0.0-alpha.11)

I thought this might be an issue with incremental builds, but running eleventy without any flags gives the same result. 🤷🏼

Reproduction steps

1. npm i
2. npm start

Reproduction URL

https://github.com/paulrobertlloyd/paulrobertlloyd-v4