Pledge payments and 'adjust payment amount' don't work with pending payments

See the video to understand this scenario.

Steps to reproduce:

1. Create a pledge (eg. $3000 in 12 instalments of $250 each)
2. Record a payment
3. Set the payment to 'adjust payment amount', select 'Adjust pledge payment schedule', and enter, eg. $500
4. Set contribution status to Pending
5. Save

Now note the that amount due is set to $0, amount paid is set to $0, and no other future payments are affected.

6. Select 'View payment' and edit. Set the payment as completed.

Now note that the amount paid reads as $500, but also that the next two months have been set as completed with $0 paid. The overall pledged amount remains at $3000 but the remaining pledges only total to $2750.

Honestly, I don't totally understand the role of a 'pending' payment, but nonetheless this behaviour seems buggy — I should't be able to get myself into a position where the instalments sum to less than the pledged amount so long as I'm selecting 'adjust payment amount'.