That would be consistent with the freezer trick fixing the boot.
And this did not happen before the new installer, right? So doesn't it mean there is something in the software or a hardware effect caused by the software, which suddenly amplifies the significance of the temperature of the router or flash chip specifically in respect of reads to the point that the router may not boot? Does UBI play a part here?
I did a little digging and perhaps one or more of the following may help:
https://www.sciencedirect.com/science/article/abs/pii/S0026271412002831
The following, taken from the first link above, seems potentially significant to me:
... and especially the last paragraph.
So might it not help mitigate temperature-related read problems to alter the way in which the flash memory is read during boot, e.g. by avoiding reading from the same block too quickly?