On rare occasions, I've seen Webflow merge a text element into an adjacent DIV or Collection List, and become a permanent, unremovable part.
The Designer treats it as a part of the element, and even performs certain layout and styling operations on it, but that text part is not selectable and cannot be deleted.
When this happens, you'll need to re-create the element that has the text attached to it.
The process is simple, but sometimes painstaking. The video here shows how to do it in the most complex scenario, which is a Collection List.
My best guess is one of two scenarios;
A data migration glitch. Over time, as Webflow improves the Designer, the underlying designer data is affected and must be migrated. An old template might be migrated improperly, in which case
I suspect that basic text nodes were at one time represented differently in Webflow, and that a change there made old templates difficult to parse and upgrade.
A save glitch. There's a lot going on in a save operation. If the network is glitchy or save is interrupted, it seems possible that wires could get crossed, so to speak.
In addition, I occasionally see this happen when I use Finsweet Unbind CMS candy - which I use a lot! Note Finsweet may have nothing to do with this, and it might be connected to the save glitch scenario.