Just to get one potential bone of contention out of the way: Duden recommends "stehen lassen" (two words) as the infinitive but accepts "stehenlassen" (one word) as an alternative. The author of the book I'm reading chose "stehenlassen" so that's what I'm using here.

And now to my actual question:

According to Duden, lassen is one of those verbs that, when used together with (an)other infinitive(s), is found "überwiegend" as an Ersatzinfinitv (here lassen) rather than as a past participle (here gelassen) in tenses that would otherwise require a past participle.

So far, so good. Now the conundrum. I have the following sentence:

"... und einmal habe er den Rest Wein ausgetrunken, den Karl Böhm stehengelassen habe"

followed two sentences later by:

"... es war zwar nur noch wenig, aber er hat ihn stehenlassen."

Why is lassen treated differently in those two sentences? Anyone have any suggestions?

Thank you!