When I open or reload https://play.ruff.rs/93f469fb-9725-411c-83c0-2c0d04d0ab75, I do not see any issues being underlined. I start seeing them as soon as I add a newline, or space, or anything else. But I'd expect them to be shown right from the first page load.
Microsoft Edge for Business
Version 126.0.2592.56 (Official build) (64-bit)
Reproduces in InPrivate mode with no extensions.