The difference is between summer and winter months. There is a difference due to the climate. Process conditions so far have not shown the improvement we need to match the physical property of parts molded in winter months where air is cooler and lower humidity.
My first reaction to this – and admittedly slightly “outside of the box” – is that the ambient air that is between the advancing melt fronts will be rapidly compressed (to an extent dependent on how easily it vents versus how quickly the air volume is reduced) and heat up. Clearly, it is not going to reach as high a temperature in winter as in summer, nor will the lower amount of moisture it contains be as likely to get entrapped between the melt fronts and potentially weaken them as they join.
In higher humidity conditions, apart from the increase in water content in the air in the tool void prior to fill, maybe you’re getting condensation at times (if the press is held-up between shots), in which case you will be pushing lots of water into the weld-line. Worth noting, also, is that even if you have the same barrel settings and water temperature and flow settings throughout the year, actual conditions that the material encounters (precise melt temperature at point of entry into the injection mold and actual mold surface temperatures) will vary, because of the R.H. and temperature differences. Hire a thermal imaging camera at relevant times and look at the readings on the parts at point of ejection. You’ll see differences, I’m sure.
There are possibilities to employ vacuum-assisted venting (by default, this would pull moisture out with the air) in the injection mold void prior to fill. Also, as I’ve seen with multi-impression PET preform molding, air-conditioning enclosures around machines.
I used to assist periodically in an injection molding shop in China, with >90% RH at >40°C (104°F) and had not been made aware of this problem. That is not to say, of course, that it wasn’t there. I cannot see that heating the pellets before use will resolve this particular problem, even if advisable for other reasons. I’m certain it’s nothing to do with the material’s weather resistance.