Why does my PD40R V3.0 downshift to a lower mode after being used for a while and the higher mode cannot be selected?Updated 10 hours ago

There are two reasons:

Intelligent Overheat Protection. The flashlight will accumulate a lot of heat when used at relatively high output levels for extended periods. When the flashlight reaches a temperature of 60°C or above, the lamp will automatically step down a few lumens to reduce the temperature.

Low-voltage Warning. When the voltage level drops below the preset level, the flashlight is programmed to downshift to a lower brightness level until Low output is reached.