This is because the 249 bot does 1 damage (see map/item_archtypes.dat).

According to lines 997-1006 of src/enemy.c, only hits that do greater than 5% of
the max target health have a chance to stun. So a 302 (with max hp of 30) cannot
be stunned by this. Bots like the 296 Sawmill (max hp 300) will rarely be stunned.

A 249 bot therefore can only stun bots with a max hp of less than 20.