The only camera brand I know that locks out the hotshoe when Sync Speed is exceeded is Pentax, though I have not specifically tested the 80D. I would suggest going back and forth between 1/250 and 1/320 to see if you can repeat the issues. However, 1/250 is as high as you would want to trigger the flash, as anything above would yeild a black bar.
Also, Canons are not freindly to manual flash by default when in Live View. You would need to disable any silent shoot modes to allow manual flash triggering in live view mode. I would suggest all testing to include use with live view off as a control (i.e., if it is not working and in live view, turn live view off and see if it works).
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