To be honest, I don't take the reports I read that seriously.
For one thing, a lot of UA larpers have had a serious bias against latex weapons with fibreglass cores, preferring "boffers" made with PVC pipes and soft foam. I think those people jump on any report of issues with fibreglass and blown them out of proportion/reality.
But also, after a long conversation with a US larper about fibreglass I realised they were using hollow fibreglass tubes with a large internal diameter, not solid fibreglass rods. I think they thought that seeing PVC pipes are hollow, fibreglass cores should naturally be hollow too. But who knows how hollow tubes compare to solid rods - they might be more sensitive to cold. The only time I've tried using hollow cardbon-fibre rods I found them extremely fragile in any temperature.
Not all solid fibreglass/carbon-fibre solid rods are the same either. They may have been using a more brittle type of rod and blamed a breakage on cold when it was actually just poor choice of material.
There are lots of fibreglass-rod latex weapons in the Nordic countries, and I don't think I've heard of problems in the cold.