Magnets are a strange thing.....striking, dropping or smacking them can cause them to lose their field or at least measurably reduce it. Magnets left unused (without any ferrous material nearby) tend to lose their strength----as a matter of fact, some magnetic base micrometers that I have use a metal plate that connects the free ends of the magnets (in the base) so that while in storage the strength of the magnets remains intact.

I suspect that some magnets that had been in a fixture or formation in combination with other magnets might present anomalies upon their removal of said arrangement, and this may be what occurred when you took the focus assembly apart. Bottom line, you had a weak magnet (or magnets) that sorta kinda worked in unison, but once you disturbed their positions then they went flooey.

That's my story and I'm sticking to it!

Anthony