We expected, that if the sales tax group on the Main account was left empty, the Sales tax group would have been fetched from the Retail store. We regard it as an error that D365FO is not able to do fetch the sales tax group from the Retail store.
We also expected that the item sales tax group was fetched from the General ledger parameters – field “Item sales tax group” in case no Item sales tax group is added to Main account > Legal entity overrides > Sales tax.

Our customer has 6 main accounts they use as income/expense accounts.
As they have stores in 9 countries, they now must create another 6 times 8 Main accounts in order to handle Sales tax correct … or make a modification in the integration with the POS, where it then picks up a sales tax group, related to the country.