I wrote "lukanya sudah dibersihkan, namun masih berdarah", but it didn't accept my answer. Actually, "tapi" and "namun" are synonyms in Indonesian.
There remains a functional, and frequency difference. Tapi = but, namun = however - in common use.
"Lukanya dibersihkan tapi itu masih berdarah." I know it's unnecessarily redundant but is it incorrect too?