I do think your situation was with brittle brass, however,

not entirely sure what causes splits. When we are forming .17AH from (even new) .22 Hornet brass, the normal 'deal' is to neck .22 Hornet down in the .17 AH seating die to .204" then use the FL die to neck to hold the .172" bullet.
Now, if the fireforming load is too light as when using 3 gr. bullseye with kapok, the shoulder will only partially form and will have splits around it's perifery.
A full power load, as used in formed brass, forms the brass perfectly, with no splits - ever.