are subject vs are subjected

Hi everyone.

I just found a sentence that make me feel confused.

“All new staff are subject to a probationary period of…”

Why we use “are subject” instead of “are subjected” since that this is in passive form, so “subject” must be in past participle?

I think it’s because subject is also an adjective.

Okay, now I understood. Thank for enlightening me :smiley: