On Dynamics 365 (online), set maximum Length on [Multiple Lines of Text], try to add the amount of values more than maximum length, and it simply cannot be added without displaying a error message.
But on Portal, try to add the amount of values more than maximum length, and the paragraph can be indented, and the error message is displayed when trying to save it.

It would be user-friendly if the values which are over the limit cannot be added and if Dynamics 365 (online) and Portal have the same behavior.