Rescheduling would be fine if they either reschedule or tell you when they’ll contact you to reschedule. Otherwise it’s too vague. Like you said red flag. Cancelled interview also would be fine as long as that’s what you were told. What’s up with secrecy.

Could you contact them and ask when they reschedule it for. If they give you bogus reply, then you’ll know not to deal with them.

I don’t recall if you tried that before but could you start applying places directly and screw these agencies. You mentioned gaps in employment but you could explain it by taking care of elderly relative and going to school. In fact taking leaves to take care of parents isn’t uncommon.

I know some people have a good experience with recruiting agencies but many don’t. It’s frustrating because it’s like you have no power. You are at mercy of these people and they don’t seem to care