Image size changes when adding caption

I got the following problem:
I put an image to a post (full size, centered) and it is correctly shown in desktop and mobile view (max-width:100%).
As soon as I add a caption (with the image-editing-tool) the size of the image is displayed with the full original size and the whole post ist stretched to the size of the image.
Any idea why this happens and how to fix it?
(Travelify Version: 3.0.4
WP 5.3, PHP 7.3, MySql 5.7)

Hey there

Sorry about that, please provide one example url with only image and one with image and caption,