The minimum contribution rule is applied to forms with separate membership and contribution payments, and NOT applied to forms with single payments. It should be the opposite.
1) Create a contribution form with membership, additional contributions, and 'separate transaction'
2) Submit form with a contribution amount less than membership. Observe that an error comes up indicating minimum payment is higher, however the transaction will be applying any contribution in addition to the fixed membership.
3) Change the contribution form do NOT have 'separate transactions'
4) Submit form and observer that a contribution amount less than membership can be submitted, even though the payment is supposed to cover the membership fee.
Suggested fix: around line 398 of CRM/Contribute/Form/Contribute/Main.php add "!" in front of the 'is separate payment' check:
if ( $self->_values['amount_block_is_active'] &&
!CRM_Utils_Array::value( 'is_separate_payment', $memBlock )