On dmaster demo:
I've added a second payment processor option - Dummy - and have made Dummy the default payment processor;
Load form: all good - payment fields appear and Dummy radio button is selected.
Next: hit Other Amount -> payment fields disappear (no $ are due)
add 1 in Other Amount -> payment fields re-appear - but no payment processor is selected.
Therefore - filling out the credit card fields - and hitting Confirm Contribution -> results in an Error message that Payment Option is a required field.
It's minor as the user can still make this work - but would be annoyed to have to fill out all credit card details/billing fields again;
Use case: switching between Credit Card and ACHEFT payment processor