Simple program to set byte 0x25 to 0x8 (saying you completed normal mode) and to recalculate the checksum of the diablo 2 Resurrection saves.
Checksum code was copied directly from https://github.com/ternence-li/Diablo2HeroEditor
Just close D2, click load, select your save and hit the other button and start D2 up again.
Only needs to be done once per save it seems.