spend: feerate can be lower than target value

[jp1ac4]

The fee of a Taproot (only affects Taproot?) transaction can sometimes be 1 sat lower than required to achieve the target feerate as defined by the coin selection algorithm, i.e. feerate = fee / (weight/4).

Note that #1132 is similar but relates to whether or not the vbytes are rounded up when calculating the feerate.

[jp1ac4]

This is possibly related to #1322.