I had same issue: Created ps1 file in share and task scheduler to run with -Bypass file \fileshare deployed with GPO under NT AUTHORITY\System to run, but it failed with permission denied, even dir \sharedfolder was showing directory. Tried many times didn't work but when I ran the script (ps1) localy it ran fine, so it has to be permissions on the share folder which it had everyone and SYSTEM as shared permissions to run and Security as well.

The fix was when I add "Authenticated Users" under the NTFS (Security Tab) on folder that was shared, taskscheduler start working.