Symptoms
When you set default sales tax group to vendor or customer, and enter vendor or customer account to a voucher, their setting will override the sales tax group information in the voucher. So, when you enter vendor or customer account to a voucher, and want to set different sales tax group to other lines, sales tax group would need to be manually modified.
Cause
This behavior is by design.
Resolution
This code which overrides the sales tax group is found in the below callstack printout that shows when the acountNum field is modified, the taxgroup data will eventually be updated to match whatever is on the vendor journal record via the updateTaxGroup() method.
[s] \Classes\LedgerJournalEngine_Server\updateTaxGroup 27
[c] \Classes\LedgerJournalEngine\initTaxGroup 90
[c] \Classes\LedgerJournalEngine\initFromOffsetAccount 9
[c] \Classes\LedgerJournalEngine\initFromVendTable 63
[c] \Classes\LedgerJournalEngine_Daily\initFromVendTable 4
[c] \Classes\LedgerJournalEngine\accountModified 93
[c] \Forms\LedgerJournalTransDaily\Data Sources\LedgerJournalTrans\Fields\AccountNum\Methods\modified 21
This behavior is by design.
[s] \Classes\LedgerJournalEngine_Server\updateTaxGroup 27
[c] \Classes\LedgerJournalEngine\initTaxGroup 90
[c] \Classes\LedgerJournalEngine\initFromOffsetAccount 9
[c] \Classes\LedgerJournalEngine\initFromVendTable 63
[c] \Classes\LedgerJournalEngine_Daily\initFromVendTable 4
[c] \Classes\LedgerJournalEngine\accountModified 93
[c] \Forms\LedgerJournalTransDaily\Data Sources\LedgerJournalTrans\Fields\AccountNum\Methods\modified 21
This behavior is by design.