Changed image sizes in the preview

For no apparent reason, image sizes in the preview and in the post are now displayed in reversed size in the blog view. Previously, a portrait-format image was shown in full size in the preview and in landscape format in the post. now it is the other way round. The preview in the blog view shows a reduced landscape format, the post page now shows the full image size. This cannot be changed by deleting and resetting the images. No other setting options were found. How can this size difference be adjusted again?

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