In the editor, by default, image blocks are 100% the width of their container, regardless of their actual dimensions. As soon as the user adds a custom width to their image however, that responsive guardrail is gone, and the image will flow off-screen.

The max-width guardrail works on the frontend. The editor discrepancy appears to be related to the in-canvas resize handles (see also).

Tested with a clean install of WordPress 5.9.
First reported over in the Twenty Twenty-Two support forums.