EDIT: After a few weeks, I have now lost the part for the first time, but fortunately only at home. After a while I found it again. At best it is likely if you tighten the knob on a regular basis. Even a small rubber ring between the button and triggers could possibly help.
EDIT: Have now added a small rubber ring around the thread and now holds the button since 8 months without rounding and intensive use. Hab genommmen a rubber ring, the one used for darts, so that the shafts keep tidy. If my opinion is equal to the supply.