In the latest release of magento 2.3.3 there is a bug in magento category thumbnail image. it means whenever we upload the thumbnail image of any category and if we update any other attribute of same category then thumbnail images are going to delete automatically.
Below are solution of this issues:
Go to this file vendor\magento\module-catalog\Model\Category\Attribute\Backend\Image.php file on line no 128 and replace this line
$imageName = $this->checkUniqueImageName($imageName);with
if (array_key_exists('tmp_name', $value[0])) {
$imageName = $this->checkUniqueImageName($imageName);
}
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