tl;dr

When an automated transaction uses a complex query to specify the rule, if a new account gets introduced in that rule, this account is omitted from the balance report.

long form

Consider this simple transaction

2024-06-18 * Payee
    Expenses    42 EUR
    Assets:Checking

A simple balance query shows

  -42 EUR  Assets:Checking
   42 EUR  Expenses
---------
        0

Now, let's assume this purchase is from a store which has the VAT baked in; but we want to know how much VAT we pay per purchase. A simple rule for an automated transaction would look like

= ^Expenses
    Expenses:Tax:VAT  0.13
    $account  -0.13

Our balance query now yields

  -42 EUR  Assets:Checking
   42 EUR  Expenses
    5 EUR    Tax:VAT
---------
        0

This makes sense… until we want to target VAT to certain postings. The ^Expenses rule no longer works. We would need some complex query to properly target these postings. One way I thought of doing this is using tags. Our ledger now becomes…

= %/vat/
    Expenses:Tax:VAT  0.13
    $account  -0.13

2024-06-18 * Payee ; :vat:
    Expenses    42 EUR
    Assets:Checking

Note the tag is on the payee line

With this, the balance query looks like

  -42 EUR  Assets:Checking
   42 EUR  Expenses
---------
        0

…no sign of the Expenses:Tax:VAT account.

When I shift the tag into the posting, like so

= %/vat/
    Expenses:Tax:VAT  0.13
    $account  -0.13

2024-06-18 * Payee
    ; :vat:
    Expenses    42 EUR
    Assets:Checking

Note the posting is now applies to the Expenses directly. The balance query now reports

  -37 EUR  Assets:Checking
   37 EUR  Expenses
---------
        0

This is what I expect — 5 EUR out of this should be in Expenses:Tax:VAT — but there's no Expenses:Tax:VAT listed in the balance report.

When I try to switch out = %/vat/ for = expr has_tag("vat"), = expr has_tag(/vat/), or any other expression which isn't the short form, I get

While parsing file "/path/to/auto-tags.ledger", line 1:
While parsing value expression:
  has_tag("vat"

While parsing automated transaction:
> = expr has_tag("vat")
Error: Invalid token '<end of input>' (wanted ')')