Steps to reproduce

1. Go to Administer Wiki > Look & Feel > Image Styles
2. Create a new Image Style (e.g. 'Style1' - choose some distinctive settings, like a specific Width/ Height, e.g 500, default Alignment: Center, but keep "Adjustable Size" not checked)
3. From "Default Style" drop-down, select the newly created style (e.g. 'Style1')
4. Check (tick) 'Force Default Style' checkbox
5. Click "Update the default image style" button
6. Edit a page (e.g. Sandbox) with CKEditor
7. Click on 'Image' from toolbar
8. Select an image from Gallery tab
9. Click 'Select' button
10. Observe the "Image Styles" drop-down selection (the previously created Image Style should be selected automatically)
11. Click "Insert" button
12. Mouse hover over the image and observe the bottom-right corner

Expected results

The bottom-right image corner does not contain anything, as the Image Style that is set as default and forced has the "Adjustable Size" set to "No" (not checked).

Actual results

The resize button is displayed on the bottom-right image corner that allows the user to resize the image by mouse dragging, even if the "Adjustable Size" setting is "No" (not checked) in Administration.

From my tests, this can be reproduced only if the default Image Style is forced.