The FM2n does not do "...things like f2.8 at 4 seconds...". Best you can measure is 1 second on its dial.
Perhaps you mean 1/4 sec @f2.8. If so, then it depends what ISO you are at. You may be outside the range of the meter.
Why not test the meter at sunny 16 and then at some normal shades and so on and see if you are having an issue.