Editing an existing pledge payment incorrectly changes other pledge payments

After editing an existing pledge payment, another pledge payment amount is incorrectly changed.

to reproduce the issue:
1) Create a new pledge for the amount 1,200 to be paid in 12 monthly installments.

2) Go to the contact's pledge tab, and click the "make payment" link to make the first expected pledge payment. Click "Adjust payment amount" and change the amount from 100 to 150" Leave everything else at their default values. Click the "save" button

3) At this point everything is fine.

4) While still on the contact's pledge tab, click the "View Payment" link next to the pledge payment from step 2.

5) Click the "Edit" button, then change the payment instrument to a new choice. Click the "save" button.

6) At this point the following 2 pledge payments are wrong.