Batteries are rated for a certain continuous current draw, and if you try drawing too much, you're going to have a bad time. Some of these flashlights can draw a ridiculous amount of power, and if you're putting a cheap knockoff 18650 in it with no internal protections, it's not going to be a fun time.

It's the same issue people had with vapes exploding. The original included batteries might only be rated for a continuous draw of say, 10A, and they're adding these crazy high wattage coils trying to get 30A from them.