Unless you have something blowing very hot air onto the DIMMs, that's unlikely. They're their own PCB, so thermal transfer from the motherboard will be minimal; air off of most heatsinks isn't likely to exceed 50 degrees (typically less). The main source of heat will always be the heat the component itself generates, as nothing else has a direct thermal pathway, and even warm air will still cool a component as long as it's cooler than the component itself - it will just cool it less. The only DDR4 I've heard of being thermally sensitive is high clocked, high voltage Samsung B-die, which apparently doesn't like exceeding 50°C, but even that takes some doing.
