i had same problem with a cooler in the memento mile museum.
This started after I changed the layout and location of the cooler a little. It was still accessible to extinguish the fire and I could manually place janitor to repair but it was never repaired manually
In the man-eating plants area of my Pointy Mountains museum, there's a botanical heater that doesn't get repaired, but when it does set itself on fire, the janitors are able to put it out.
Deleting the botanical heater and replacing it makes it so they can repair it normally. (I'm assuming this will be the most helpful piece of information in this description in tracking down the cause.)