Custom Video Mode, Classic Error

It’s best to set the Video Mode at the start of a project. Or you can set it using Settings, then choose a preset. There are several 4k presets to choose from.
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When you made the custom video mode, perhaps you didn’t flip the aspect ratio to 9:16.
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That all depends on how many adjustments you want to make. Do a file Save As (give the project a new name and experiment to see if you need to start over. Various if not all filters may need to be adjusted.