I just discovered that Jira Cloud dashboards have the new “Add a Gadget” sidebar (instead of the previous popup) which renders all available gadgets and their thumbnail images. The problem is that all thumbnail images are cropped as squares with a full height but a reduced width. As a result all thumbnails look broken (even the default Jira ones) as previously they had width > height.
This breaks the rendering of app dashboard item thumbnails as well (as in our case the logo is cropped). It could be fixed by providing a square thumbnail image for dashboard items in the app descriptor.
I could not find any announcement about this change and do not see any dashboard item thumbnail format description where it is mentioned that it should be a square image.
Atlassian, please provide the clarification about this change and update the documentation.