metadata silently dropped when using both metadata_map and payer_tag/payee_tag

[zacchiro]

ledger2beancount configuration:
------------------------------------------------------------------------
# mapping of ledger metadata key to corresponding beancount key
metadata_map:
  x-payee: payee
  x-payer: payer

# metadata tags (*after* above mapping) used for specific purposes
payee_tag: payee
payer_tag: payer
------------------------------------------------------------------------

ledger-cli transaction:
------------------------------------------------------------------------
2020-12-21 * salary
    ; bank-label: foo bar baz
    ; x-payee: Zack
    ; x-payer: Universite de Paris
    Income:Salary:Univ-Paris
    Assets:Cmut:Checking                      123.45 EUR
------------------------------------------------------------------------

desired beancount translation:
------------------------------------------------------------------------
2020-12-21 ! "Zack" "salary"
    bank-label: "foo bar baz"
    payer: "Universite de Paris"
    Income:Salary:Univ-Paris
    Assets:Cmut:Checking                      123.45 EUR
------------------------------------------------------------------------

current beancount translation
(not how the value of x-payee "Zack" has disappeared)
------------------------------------------------------------------------
2020-12-21 ! "Universite de Paris" "salary"
    bank-label: "foo bar baz"
    Income:Salary:Univ-Paris
    Assets:Cmut:Checking                      123.45 EUR
------------------------------------------------------------------------