Why do LEDs in simple battery circuit flash?
Posts suggest flashing is not a recognised failure mode of LEDs.


Wrong. Flashing, or intermittent connections causing the led to turn on and off, is a typical failure mode of leds which are cheaply constructed, physically damaged, or forced with a higher current.

When some die, the rest of your parallel leds will get a higher current  which will lead them to die as well. As this happens, the leds can go through thermal expansion which breaks the internal connection then cool which causes the connection to reconnect, then expand, etc. Bond wires can do the same. Or internal shorts of the led substrate material. This can also lead to leds flashing between different brightness.