If you have enter a transaction involving two accounts with different currencies (for example a checking account in USD and an expense account in EUR) things work fine if the register is in "Basic Ledger" mode and splits are not expanded.  Suppose you are in the register for the checking account and enter a transaction with the other account being the EUR expense account.  When you enter the transaction you get a chance to specify the exchange rate and the split for the EUR account gets created using this exchange rate.

Now if you do the same thing with splits expanded (any mode other than "Basic Ledger" or just hit the "Splits" button) things don't work right.  If you enter the same transaction (one split a credit to the USD checking account and other a debit to the EUR expense account) you never get a chance to enter the exchange rate.  If you look at this transaction in the checking account it looks ok, but in the expense account both the value is zero and it doesn't affect the running balance.

Ideally you should get the exchange rate dialog when you enter the split targeting the other currency.  If this is not possible, then this sort of transaction should not be allowed.  It is wrong for GnuCash to be creating an invalid transaction.

The splits in the created transaction are 

  <trn:splits>
    <trn:split>
      <split:id type="guid">1f5ab32ce4a57fb3b380c1c88d3701d9</split:id>
      <split:reconciled-state>n</split:reconciled-state>
      <split:value>7500/100</split:value>
      <split:quantity>0/100</split:quantity>
      <split:account type="guid">46587f1c8a570091edc0021d5c1eaf4e</split:account>
    </trn:split>
    <trn:split>
      <split:id type="guid">e268892e68eb9b0982fccf33ad98ab0d</split:id>
      <split:reconciled-state>n</split:reconciled-state>
      <split:value>-7500/100</split:value>
      <split:quantity>-7500/100</split:quantity>
      <split:account type="guid">ab40d43d2091ea21672ec1ba2113b630</split:account>
    </trn:split>
  </trn:splits>


Note that the quantity in the Euro expense account is zero.

This is in SVN r13887.