I agree with this sentence :) still super rare to use though but makes the most sense.
For that example: 'I returned because I remembered you', implies perhaps, a previous memory and you are stating the reasoning behind showing up (via in-person communication) or could be a present thought when you are in some area and thinking this phrase without any form of verbal communication.
Either one works IMO.
I don't usually hear 'return' nor 'remember' in the present tense under this context. I'm thinking about if person A goes out of the house to an event and then just remembered he was supposed to take person B. So he comes back home and says this sentence:
"I returned because I remembered (I'm supposed to take you, too)" or what's more natural for me is "I came back because ..."
In written Indonesian it would be strange to use "saya" and "kamu" in the one sentence. In spoken, anything goes and there aren't really any rules, but it would probably be more common to see "aku" used in place of saya. (tbh it would be better if Duolingo just focused on using standard formal Indonesian at the beginning of the course like this, as that is what every single university course and the best private language learning institutions in Indonesia do!)
This English sentence seems to be missing verb tenses. Using past tense for both verbs would be a little more natural: "I returned because I remembered you".
Also, most English speakers would prefer "came back" instead of "returned" in this context. "I came back because I remembered you" would be closer to a natural English sentence... though even still, I don't think most English speakers would use the phrase "remembered you". I'm actually a little fuzzy on the intention behind that phrase.
Here are some English sentences that have a similar meaning but would be used in everyday speech by a native English speaker:
"I came back because I was thinking of you."
"I came back because I missed you."