got 100 splinters, how do i "create" a breachstone

^ That's the exact reason it works this way. Everyone who is running into this "Issue" is seeing it because they are storing their splinters in the 5 blank slots at the bottom of a currency tab. They cannot automatically combine into a stone, because those slots allow stacking up to 5,000 or whatever.

Maybe they can auto combine once outside currency tab

OR

have a tooltip somewhere that explains the situation

OR

at least make an announcement somewhere

OR

do nothing and let people google it themselves if they want to know.