DIAGNOSIS Yeah, this one's common. The bevel lock knob isn't the culprit. The problem is usually the pivot bolt or the detent mechanism under the shoe (base) has gotten gunked up with sawdust or worn.
FIX Unplug the battery. Loosen the bevel lock and tilt the saw to 45 degrees. Look at the pivot point where the shoe meets the saw body. Clean out all the compacted sawdust you can see with a brush or compressed air. Then, apply a drop or two of light machine oil (like 3-in-1 oil) to the pivot bolt and work the bevel back and forth. Tighten the bevel lock firmly, but don't hulk out on it—it's a fine thread.
IF THAT DIDN'T WORK The detent plate (the little metal piece with notches for 0 and 45 degrees) might be bent or damaged. You can inspect it by removing the shoe, which involves taking out a few screws. If it's bent, you'll need a replacement part.
HEADS UP Regularly blowing the sawdust out of the bevel mechanism after use will prevent this from coming back. It's a maintenance thing, not a design flaw.