It’s good to bring stuff like this to future builder’s attention for sure! At the same time, its important to recognize that washers in aviation applications are (almost) always a “season to taste” thing - add or remove (nor more than three) as required to make the fit work out right. Generally don’t assume that if the drawings who’s one, you can/should only use one… there is enough variation in bolts to require assembly with an extra washer, thick or thin sometimes.
And on the lighter side - what is it with Canadian AME’s and washers?! In the US, we are taught to always use a washer under a nut, and under the head of the bolt of the bolt is supposed to be turned. Our DreamTundra was assembled by a Canadian AME a long time ago, and there were NO washers under any of the nuts. The drawings (Canadian) showed no washers anywhere…. Last week we were resealing the IO-540 that came from a Canadian airframe company, and the engine had been maintained as a certified engine (with meticulous logs) while they had it…and there were several Adel clamps and the like (nothing critical) that didn’t have washers under nuts….. Just giving my friends north of the border a hard time - I’m sure there’s nothing systemic going on here….