Ah yes, in which case I think there could be a bug here?
When I select an existing transfer transaction in a transaction list, the transaction “type”(? - Standard/Split/Transfer/Shares, at the bottom) is selected as “Transfer”, as expected. Modifying the date, changes both sides at the same time with no warning.
However, when I select a Shares transaction, in either the Investment account, or the Cash account, it shows as Standard, rather than Shares, so it isn’t possible to modify them both together and the warning shows.
Also, while a transfer transaction has the double arrow icon in the transfer list, there’s no indication that the Shares transactions are part of a grouped transaction?