bugs - Why does x++ behave strangely under Defer?

Bug introduced in 10.4.1 or earlier and persisting through 11.1.0 or later

Using x++ or x=x+1 inside Defer generates strange results.

Evaluating the following commands one by one (in three different input cells), with Shift+Enter, one gets:

x=1;

Defer[x++];

x
(* 15 but I have also seen 17! *)

Even stranger, if one creates a notebook with only those same commands, restarts the kernel, and does Evaluation ► Evaluate Notebook then the command x returns 1 instead of 15.

Any explanations?

Comments

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