This question has come up in other forums before, some of which are lost to time. From memory, this is an oracle game, in the sense that "some of the rules are intentionally up to the group to interpret", as an intentional nod to how some rules were under-specified in older D&D and other TTRPGs. So the official answer as I recall it is "you tell me!"
When I run the game, if someone rolls worse than their current HP, they keep their current score.