Additional things to try for an older version of Tunic:
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For Microsoft Store, download the Tunic.ultrawide.v1.1.MS.Store.Game.Pass or the 1.0 patcher and patch both files as guided by the tool.
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For Steam and GOG, launch the game at least once, then quit and go to HKEY_CURRENT_USER\SOFTWARE\Andrew Shouldice\Secret Legend\ in the registry (Windows Search -> regedit) and correct the decimal Resolution Width and Height values, then set the Fullscreen mode and Use Native values to 1. Make sure to modify the non-Default values.