There seems to be a problem with the trigger "When a purchase document approval is requested (V3)" that should trigger when launching the action "Send approval request" from a purchase document, the trigger does not work if the purchase document type is Invoice. It works only if Document type is Order.
i have made a very small Flow, just to test the trigger.
In the trigger i deleted all conditions (normally I would add at least the condition Document Type = Invoice"
And I added an action to send me a mail.
The Flow should run both on an purchase invoice or on a pruchase order, because I did not specify the Document Type conditon in the trigger, and both are purchase documents.
If I do the test from a Purchase Order it works immediately.
If I do the test form a Purchase Invoice, the Flow is not triggered at all.