Dear Marc,
Thank you once again for alerting me to this helpful post. If I have understood what you have written, your default lme model specification is
follow-up y ~ group*y0 + time*treat + (1| id)
whilst Jorge’s M4 (which you seem to endorse) is
follow-up y ~ time*y0 + time*treat + (1| id)
In M4, an interaction between baseline outcome and time is specified presumably because the associations between baseline and follow-up outcomes wane over time.
Could you kindly clarify on the rationale for including group*y0? Are we assuming that the between-group differences vary as a function of baseline outcome?
Thank you in advance for your guidance.