As mentioned above some thermal issues were discovered on LED modules, so I decided to take a closer look with a thermal camera. Out of two modules 400nm one looks worse since LEDs are packed so close together. The main problem is as I see it that pcb does not conduct heat to heatsink. Specific thermal conductivity for FR4 is around 0.3W/mK, while depending on substrate for ALU PCBs is in the range of 2-4W/mK. Making new versions on ALU PCBs will solve the issue. I might also consider arranging(spacing out) LEDs to improve thermals further.

*Left: PQ2N-400nm module Right: LTPL-365nm module.

Image below shows the detailed construction of the LED module. You can see thermal conductive pad(greenish stuff) and thermistor wrapped in kapton between it and PCB.