X11 did not originally have the concept of a clipboard, it copied selections called "primary" and "secondary" and an application notified the server when it had a selection, and another application asked for the current selection when it was told to paste (traditionally by middle mouse click, but in terminal shift-insert does the same) and the server passed the request on to the application with the current selection. The selection was never "copied to a clipboard", either by user action or software. Thus large selections were copied between applications without another intermediate copy. But the selection had to remain in the application it was coming from, de-selecting as you did removed anything to copy and so your shift-insert did nothing.

As the windows ctrl-c and ctrl-v protocol became popular an additional "clipboard" was added so that content that was "copied" by ctrl-c could be advertised under a different name and the selection did not have to remain. Then an application where the user did ctrl-v (or ctrl-insert) could ask for the "clipboard" instead of the "primary" selection so that the original behaviour was also maintained.

So the behaviour you see is normal.

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