Double pole breaker partial failure?

Yesterday I was connecting a large toroidal transformer to an unused 240V 15A circuit in my apartment (US split phase) for testing. I plugged it in and I heard the breaker trip after a few seconds.

I wasn’t expecting that much magnetizing current so I probed the outlet with my DMM and got a very strange reading—80V line to line and 40V between both lines and ground.

I was worried the 30+ year old breaker might have "partially" tripped (presenting a fire hazard) so I didn’t think to test it with my low-impedance meter. I immediately shut off the breaker by hand and re-tested the outlet, which now read 0V (so I don’t think it was ghost voltage anyway).

Does this sound like the breaker is faulty and partially tripped? I doubt it’s ever tripped in its life as it’s normally used for the in-wall air conditioner. Are there any other potential scenarios that could cause the breaker to act in this way?

I plan to redo the test later today and if it fails in the same manner I’ll contact the landlord to have it replaced, but I want to make sure it really is faulty first.

If it is indeed faulty and I’m only using it under supervision with proper fusing on the DUT and current monitoring, are there any potential risks to using it while we await replacement? There are no other devices on this circuit; the only other 240V device is the stove and that has its own 40A circuit…so unless the breaker has some internal resistance I can’t see any problems.