Indeed it looks inconsistent as anyway we should not enter an external url in the menu item field.
We may be able to change the menu item behavior to use the same field type as the module (from text to menuitem) BUT I am not sure this would be B.C.

The issue with B/C will be that currently we store the full URL with that field, and the menuitem field would only store the ItemID. Thus you can't just change the fieldtype. Also when you change that, a user who opens and saves the item without changing anything would loose the redirect URL for sure.

So what we could do is add a new menuitem field with an additional option "- Use Custom URL -". If that one is selected the current field will be shown. If a menuitem is selected in the new field, the existing field can be hidden with showon.
That approach should be B/C but needs additional checks in code everywhere we check for that redirect URL.

I need a same field for a thrid module, maybe i could make a PR, just need to know the right way:

• Did we need to create a new form field "\libraries\cms\form\field" ?
• What could be the new name ?
• What could be the new value for additional option "- Use Custom URL -" needed for showon ?

<field
    name="login"
    type="menuitem"
    disable="separator,alias,heading,url"
    label="MOD_LOGIN_FIELD_LOGIN_REDIRECT_URL_LABEL"
    description="MOD_LOGIN_FIELD_LOGIN_REDIRECT_URL_DESC" >
        <option
            value="">JDEFAULT</option>
        <option
            value="-1">MOD_LOGIN_FIELD_LOGIN_OPTION_CUSTOM</option>
</field>
<field
    name="login_custom_url"
    type="text"
    label="MOD_LOGIN_FIELD_LOGIN_REDIRECT_CUSTOM_URL_LABEL"
    description="MOD_LOGIN_FIELD_LOGIN_REDIRECT_CUSTOM_URL_DESC"
    class="inputbox"
    showon="login:-1" />

Finally this has been resolved by #10249