I think that the actual answer is: there is no rationale, it's a bug.
I can't think of anything else, other than the possible reason I came upon with earlier which was pointed out as wrong anyway. It might have been something introduced when we were doing some updates to the Pd GUI. If I find anything more, I will respond to the Github issue.
Anyway, I am just curious about that the rationale is for starting from “untitled”, instead of last saved name? Cause to me it doesn’t make sense at all.