Pledge's 'Edit scheduled amount' does not obey 'Adjust pledge payment schedule'

See the video to understand this scenario.

Steps to reproduce:

1. Create a pledge (eg. $3000 in 12 instalments of $250 each)
2. Select 'Edit scheduled payment'
3. Edit the amount to, eg. $500, and select 'Adjust pledge payment schedule'
4. Save

Now note that future payments are unaffected and the total amount pledged has been increased — this is the opposite behaviour expected for 'Adjust pledge payment schedule'.