From the wl_pointer.enter event in wayland.xml:

When a seat's focus enters a surface, the pointer image is undefined and a client should respond to this event by setting an appropriate pointer image with the set_cursor request.

imv doesn't have a single wl_pointer_set_cursor() call in its codebase from a quick grep.

Originally reported here: https://github.com/riverwm/river/issues/616