MacBook: Disabling Messages in iCloud for all Devices doesn't affect iPhone logged on the same iCloud ID

Thank you. Sorry if I didn't give you the reply you expected.

We can't guess which software versions you have installed on your iPhone, etc. unless you tell us. (As Apple users, just as you are, we have no access to your accounts or details.)

If the operating system (eg: MacOS, iPhone iOS) version is not up-to-date, updating it will install the latest bug-fixes, security updates, and more. That action may or may not solve your issue.

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Re: "But there is no effect on the toggle button showing in iOS Settings > iCloud > Messages > "Use on this iPhone".

Any one else has explanation for this unexpected behavior."

If all your devices have the latest software versions already installed:

When you restart your iPhone, it will likely re-establish the connection and the "All devices" setting changes you made should update on iPhone.

If, after updating software and / or restarting the iPhone, you still see the toggle button is on:

Does the iPhone continue to use the on setting, or the off setting ?

Else: You may want to contact iCloud support for an explanation

Cloud - Official Apple Support

or

You may just want to let Apple know : Feedback - iCloud - Apple

Re: "MacBook: Disabling Messages in iCloud for all Devices doesn't affect iPhone logged on the same iCloud ID"

Keeping the Operating System Software up-to-date on all computers and devices can often sidestep issues.

Eg: Wondering if the iPhone's software is up-to-date with the newest compatible iOS version for that model:

You can check in Settings > General > Software Updates

(Reminder: Is always wise to back-up iPhone first, if you choose to update the iOS version)

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Else: You might try restarting your iPhone : (If needed: See how to: Restart your iPhone - Apple Support)

A restart deletes temporary files, re-establishes connections, may update the setting change you made, and often restores order if something is not working as expected.)

Thank you kindly for letting us know that the software version,

or the need to restart the device, is not the cause of the problem.

Good to be able to rule that out!

Perhaps the toggle button remains on just so, (even though Messages aren't synced to all your devices now), you may be able to check any existing iPhone messages stored in iCloud: https://www.icloud.com/

or

If that option exists: You may want to toggle that option off yourself.

(If needed: You can: Sign in and use iCloud.com - Apple Support :