> In delete-seletion-mode, trying to self insert in read-only text when
> there is a selection signals an error that clears pre-command-hook.
> 
> emacs -Q
> 
>     (delete-selection-mode 1)
>     (insert (propertize "12345" 'read-only t))
> 
> Select read only text with C-SPC + moving. Press a self-inserting key ("x").
> 
> => pre-command-hook is cleared.
> 
> delete-selection-mode does check buffer-read-only, but it does not
> protect against read-only text.

Does the attached patch fix it?