Changing debtor distribution

After entering all expenses and then using the "simplify debts" option I usually contact all people who get money and tell them who they get money from. I noticed that after coming back to this after some time the dstribution of the debtors often changed like Debtor A was distributed to Creditor X and then suddenly to Creditor Y.

Also settling up one debtor often causes to change the whole balance how the debt is distributed.

Is this a bug or am I not using the program as intended?