Aluminum electrolytic capacitors have higher ESR at colder temperatures, and lower ESR at higher temperatures. That is my guess at what went wrong. They generally perform better at higher temperatures (within reason), although a particular circuit may require the ESR to be within a certain range (don’t go randomly swapping out your capacitors for low-ESR capacitors, it may make the circuit unstable or destroy something).

Yes - the circuit had electrolytics on both the input and output side, and for my analysis I assumed they were both perfect, zero ESR capacitors, which is clearly an over-simplification...

When I did the stability analysis, it was marginally stable (ie. poles on the axis). I would guess whoever designed this lamp fitting used the wrong formula or didn't understand stability analysis.

By chance, it was mostly stable when warm (presumably because some resistance or capacitance slightly changed to push the pole to the left half plane), but when cold it flashed on and off at about 2Hz.