This is a known issue we have in the inline edit mode. The incell and popup edit modes don't exhibit it.
A potential workaround for the time being (apart from using a different edit mode) is to use an empty custom edit template.
I made this thread public (I changed the title a little so it better explains the situation) so everyone can track the status of this bug. Here is the new link for you when you get this as an email notification: https://feedback.telerik.com/blazor/1408224-editable-false-does-not-work-for-grid-columns-in-inline-edit-mode.