I believe the behaviour described below was introduced as of 4713d
d425beac5cb459704e67dcb8f6faf714375 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