I've noticed that new transactions can sometimes be shown as being "via Legacy". Some examples can be seen on T5141. I'm not sure what causes this but it doesn't seem like the intended behavior.
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T3904 is a duplicate but this task is a little more descriptive about the remaining issues.
These "Legacy" transactions are being generated by Maniphest's edge event listener. I think edge event listeners were a technical blunder on my part, and we should move away from them. We could fix this by changing the content source to "Web" (which is probably true in almost all cases) but a better solution would be to move this logic out of event listeners and into TransactionEditors.