[GNC] Budget report fails in multiple currencies

[ml enquirer]

Dear all,

I spend in two currencies and, historically, I chose to create separate
accounts under each expense line for each currency. In future I will do
this differently, employing trading accounts, but what I'm about to
describe should still work. I budget in my 'home' currency, and the budget
report would combine the "sub-spends" in each currency (using the price
database).

However, the "Actual" budget totals don't behave as I expected. I've set up
a simple scenario to explain:
1) I set up an assets and expense ("Groceries") account
Under each I set up a "GBP" and a "EUR" account
3) On 1st Jan of 2020, 2021, 2022, 2023 I spend 10,000 EUR and 10,000 GBP
on "Groceries"
4) I set up the price database with four entries, on 1st Jan:
 - 1/1/2020 1GBP = 1EUR
 - 1/1/2021 1GBP = 2EUR
 - 1/1/2022 1GBP = 1EUR

I've attached a simple reproducer ("bugreport.gnucash", though perhaps this
is not a bug so filename might be inappropriate!).

When I look at the default budget report, I see that the total per year is
always 10,000 of each currency in the sub-accounts, as expected, but the
budget's "Act" total under "Groceries" is:
1/1/2020: £15,000 [I expect £20,000 because EUR=GBP on 1/1/20]
1/1/2021: £15,000 [I expect £15,000 because 1EUR=0.5GBP on 1/1/21]
1/1/2022: £30,000 [I expect £20,000 because 1EUR=1GBP on 1/1/22]
1/1/2023: £20,000 [I expect £20,000 because 1EUR=1GBP on 1/1/23]

Can someone explain where my naive expectation is wrong? Is GnuCash doing
some strange interpolation between exchange rate values?

Many thanks in advance.
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