prettier fails to parse now required less format for at rule variables

Prettier 3.9.6 Playground link

Input:

@media @{a} {
    .a {
      color: red;
    }
}

Unexpected Error:

SyntaxError: CssSyntaxError: Unknown word breakpoint-regular (13:18)

Why?

using @a less now prints a deprecation error:

DEPRECATED WARNING: A bare @variable in an at-rule prelude is deprecated. Use @{variable} interpolation instead.

using @{a} prettier gives a error.