We are experiencing weird behaviour using the Turnitin LTI mixed with the Feedback Release Date - in various combinations of whether assignments are manual/automatic grade release, muted/unmuted once they have submissions, etc.
We have come to the conclusion that the neatest fix would be for canvas to have an additional field in the Assignment date settings with the same name as Turnitin use - Feedback Release Date. This would hide grades for both the Speedgrader-driven assignment comments and the TII one (the TII LTI currently sends the grade back to canvas even when you set the grade before the Feedback Release Date set on their LTI). It would also give institutions one of the TII 'features' using just Speedgrader, if they don't have (or can't afford) a 3rd party Assignments LTI such as Turnitin.
We are aware you can have a Grade Posting Policy of Manual or Automatic in the Grades view, but this is an extra step for a teacher (plenty of home struggle with technology as it is).
If implemented, we would expect this feedback release date field in canvas to populate the same date/time value into the Turnitin LTI's field for this, if that LTI was added as an External Tool for that Assignment, for consistency.