Thanks @kornfreak28, Ricochet and wolfemaaan for your advice on this!
In the end, I fixed it in the stupidest possible way.
I took out the left ramp so I could have a closer look at the gate underneath. Everything looked fine topside. Unscrewed the assembly from the bottom of the playfield and let it hang. Yup, every time it activated, the magnet refused to let go. Once forced to release, there's no residual magnetism, but ones it activates it'll just stay that way.
So I stuck two pieces of masking tape between the magnet and the plate. That was just enough to dull the magnetic field enough to allow it to let go of the plate. A hack, for sure, but it should do the trick at least until the tape wears through.
I think what happened is that the metal plate became slightly magnetised, so once it connects it's too strong to let go. If I had a way to demagnetise the plate, that might be a more permanent solution.
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