I am fairly sure this can’t be the explanation as what would matter would be pressure rather than density.. a) The air is less dense (ie it becomes lighter) in hot weather, which leads to a reduced 'punch' in the cylinder as the piston and washer drive home.
I wonder if the explanation is that the spring constant of your spring is decreasing slightly with increased temperature. However that would give a linear effect and not the tipping point you describe.
I can only hypothesise that some of the airgun fairies who live in your omega refuse to work above 25 degrees. I am afraid you must have a unionised gun, which will reduce its value considerably