Running with a stack build of up-to-date master.

Transaction modifiers seem to interact oddly with inferred amounts. The following works as expected:

= ^expenses:groceries
    [budget:groceries]                           *-1
    [assets:bank:checking]                        *1
10/7 * MARKET
    expenses:groceries:food                      $20
    assets:bank:checking

$ hledger -ffoo.ledger reg --auto
2018/10/07 MARKET               ex:groceries:food              $20           $20
                                [budget:groceries]            $-20             0
                                [as:bank:checking]             $20           $20
                                assets:bank:checking          $-20             0

But the following does not:

= ^expenses:groceries
    [budget:groceries]                           *-1
    [assets:bank:checking]                        *1
10/7 * MARKET
    expenses:groceries:food
    assets:bank:checking                         $-20

$ hledger -ffoo.ledger reg --auto
hledger: could not balance this transaction - can't have more than one balanced virtual posting with no amount (remember to put 2 or more spaces before amounts)
2018/10/07 * MARKET
    expenses:groceries:food
    [budget:groceries]
    [assets:bank:checking]
    assets:bank:checking                $-20