I believe the behaviour described below was introduced as of
4713dd425beac5cb459704e67dcb8f6faf714375
by Noam Postavsky on the 15'th of March 2017.

If you set ‘tab-always-indent’ to 'complete in the SLIME REPL buffer, then
type an opening parentheses followed by a few letters and press TAB, the
cursor will move to column zero and display an error message: “Text is
read-only”.

This happens in the function ‘lisp-indent-line’, and I believe it was
introduced in the change that I have mentioned above.

Regards,
Elias