I am in a similar case, but I managed to find "why"...
I use crab toss daily to farm the win and I chain 10 win a day to progress the "Crab Toss Champion" achievement.
I'll have a normal progression on this achievement, and in 10 matches gain 10 tick for the achievement.
But at the same time I only got 5 "Splendid Chest" and progression toward "Crab Toss Regular"
And 5 times nothing
And the reason is: having the crab in hand at the end of the match
When I win with the crab in my hand when the timer reach 0, I get the chest,
when I win without the crab, I get nothing.
So it's quite annoying, as I need to plan everything to be able to get and secure the crab in the last seconds of the match and a single mistake means I don't get any reward.