@needs native check

прибыл на вершине горы is wrong, but I cannot compare with the originals to correct

очень удивительно is a bit not-Russian, more natural would be очень странно

In English: "I've just arrived at the top of the mountain. That's really surprizing, because according/due to my map there should be a lake."

right, but Russian прибыл demands the accusatif

(In order to avoid misunderstandings:) So what overall solution do you propose?

Я только что прибыл на вершину горы, что очень странно: согласно моей карте, здесь должно быть озеро.

I believe, some German propositions might be excused from translation in favor of the logic - otherwise the sentence becomes too much fragmented.

I think that's right. Даже без "потому что" причинная связь ясна.

In my opinion, "прибыл на вершину горы" sounds a bit unnatural in Russian. We'd rather say "достиг вершины горы", "поднялся на вершину горы", etc. On the other hand, the context of the original sentence presumes a certain surprise since arriving at the mounting top was quite unexpected. In this case, "очутился (оказался) на вершине горы" looks more appropriate.

Thank you for your sharp-toothed analysis. ☺

I prefer "достиг вершины горы", because I think that our friend is in the first moment really proud that he has arrived at the top. Much of the nice absurdity of the situation whould be lost, if I would right from the beginning present the "conquest" of the top as a bad thing, a failure. Что ты думаешь об этом?

well, I think it's a good choice :-) Besides, you're the author of the sentence so you may have a better point of view. ;-) Joking apart, I think that one of the most difficult things about Tatoeba is that many sentences look taken out of context so it's hard to make proper translation sometimes. :-)

Absolutely, достиг is the best choice.

"... one of the most difficult things about Tatoeba is that many sentences look taken out of context ..."
That's 100 percent right :-( (at least 100% :-D)

Большое спасибо вам обоим!