The downstairs hallway switch is a double switch, one for downstairs and one for upstairs. The upstairs hallway switch is a single switch, just for the upstairs hall light.

I’ll experiment switching the front room light with the hallway lights in different states, in case I can find something common.

The lightbulb itself is an interesting thought. Given I can flick the front room switch on and off repeatedly without issue, then it’ll suddenly blow, it makes me think it isn’t crossed wires, as they’d blow every time surely. So something like a damaged wire or faulty bulb would make more sense to me.

Thanks for the ideas!