Read this, it happens when the X5/X6 driver is used for high drain low voltage triples (it was designed for a single relatively high voltage XP-L), and yes, using an 18650 can make a difference from using an 18350.
Also, the X5/X6 driver when used in high drain flashlights is almost impossible to set up with a lighted tail without messing up the user interface.
The BLF-A6 driver is miles less sensitive to its electronical surroundings (probably the Attiny13 in the A6 driver is more robust in this than the Attiny25 in the X5/X6 driver), no wonder that Astrolux sticked to that driver in all of their EDC models.