New Bedding AFCI started tripping a few weeks ago after a rain. It trips after a couple hours of being reset.

Electrician replaced breaker and unplugged everything, Told us to plug 1 thing in every day until it tripped. Plugged in items in this order 24 hours apart
Malibu Light
Microwave
Small Frig
Fireplace in morning
Came home at night and tripped
Unplugged fireplace still tripping but not right away

This is all in a Bedroom? Where are the Malibu lights? If connected outside, should be in a GFCI, not an AFCI. They are very sensitive to water.

If this is an older house, built before ArcFaults, it may have something using its neutral, or vice versa(Circuit wasn't totally isolated when Arc Faults installed). The Hot and Neutral must draw equal current, as with GFI's.