I worked in a tech company as an SWE intern summer last year, and have returned to the same workplace in the capacity of a full-time engineer now. My intern-time project was on a completely different line (included ML based libraries and skills such as time-series forecasting that I picked up) while my present work is totally Python-based tooling development. Should I list these two experiences separately in my resume, given the time-periods are different as well?
Listing them separately is generally what I see people do since the responsibilities and timeline of internship and full-time generally have very little overlap.
Yep, I have never seen anybody merge their internship and full-time experience together. Keep them separate.
This makes even more sense for you given that you aren't really extending your intern project as a full-time entry-level engineer and are working on a different scope and stack entirely.