The reason that your request is not adding the line when you are setting the Hold to false is that most of the time the field are processed in the order they appear on the screen.
Since you are setting the Hold to false the record immediately goes to the Pending Approval status and cannot add any more information to it.
This is the limitation that you can achieve using a PUT request
I would recommend that you use a POST request instead to trigger the action “ReleaseFromHold”
according to the following article
https://help.acumatica.com/(W(19))/Help?ScreenId=ShowWiki&pageid=f01f3b8b-56c4-48ab-a043-dcbb10e3bfb3
in the entity section you can put you entire Journal Transaction entry and it will process it and save it before attempting to trigger the action which would remove it from the Hold status and into the Pending Approval that you want.