There is a case in which gnucash 1-8-branch fails adding a new split to a txn.
It's when you are editing a multiple currency txn from the account that isn't
the principal currency one.

Reproduce it this way:

Starting with no files, create the accounts dop and dop1 with currency dop, and
the account usd with currency usd.

Now enter usd register and add a txn involving dop (remember to assing the
exchange rate).

Close usd register and open dop register: now split the txn, and add a new split
involving the dop1 account: you'll see that the split is saved, but there isn't
a way to save the split amount.

If now you go back to the usd register, from there you can put an amount to the
dop1 split. Unfortunatly, it doesn't appears when you open the dop1 register.
Inded, the amount of the dop split has desappeared too.