The JIT is configured through two settings whose interaction is not obvious, and the most common outcome of enabling it is that it is not enabled.
opcache.enable=1
opcache.jit_buffer_size=100M ; 0 = JIT OFF, whatever jit says
opcache.jit=tracing ; or 1255, the numeric form
; and the verification, which is the point
; php -r 'var_dump(opcache_get_status()["jit"]["enabled"]);'A buffer size of zero disables the JIT entirely regardless of the mode, and zero is the default — so a configuration that sets only opcache.jit does nothing at all. The numeric form is four digits each controlling a different behaviour and is impenetrable without the manual; tracing and function are the two aliases worth knowing. Verifying through opcache_get_status is the only way to be sure.