It was an issue with the main JPEGXL library being unmaintained and possibly open for security flaws. Some people got together and wrote a new one in Rust which then became an acceptable choice for a secure browser.

Unmaintained? You must be mistaken, libjxl was getting a healthy stream of commits.

The issue was the use of C++ instead of Rust or WUFFS (that Chromium uses for a lot of formats).