Magento2 – Category Thumbnail image not uploading

I am using an extension of Amasty for layered navigation which also provides facility of category thumbnail image upload.

Unfortunately the upload of Thumbnail is not working for me, I am not getting any error in console and also nothing is received in var logs.

On debug I figured out few things:

1) when uploading Thumbnail image from category details page afterSave function is not called. I found this function in

magento/module-catalog/Model/Category/Attribute/Backend/Image.php 

2) When uploading simple Category Image that comes default this same afterSave is called.

3) Due to this I think “moveFileFromTmp” this function is not called that is in

magento/module-catalog/Model/ImageUploader.php 

This function is responsible for moving file from temp folder to category folder

4) Image is uploaded successfully in temp folder of category.

5) I have crosschecked in Database and the value in backend_model for the attribute is already

Magento\Catalog\Model\Category\Attribute\Backend\Image 

** Any suggestion what may be the cause of this uncertain behaviour.