Simple Past...
1: I lost it, but then I found it
...is a valid, natural utterance. But...
2: I have lost it, but then I found it
...isn't idiomatic, because Present Perfect implies an action that continues (or at least, continues to be relevant) up until time of speaking.
But since the thing has been found1 after being lost, obviously the (act or effect of) loss doesn't continue up to time of speaking. So we'd expect Past Perfect I had lost it - referring back to the earlier action before I found it.
We don't say...
3: I haven't lost my key because I found it
...but we do say...
4: My key isn't lost because I found it
...or more often, just...
5: My key was lost but I found it
1 Note that Present Perfect is fine here, because the (act or effect of) finding the thing continues up until time of speaking.
The thought you seem to be trying to express is 'Hey. There my key. I thought I'd lost it!'
The meaning is very broadly the same, though your example depends as much on context as grammar.
'I haven't lost my key because I found it' is wholly comprehensible, but it's too unusual to be useful.
– Robbie Goodwin Dec 24 '23 at 18:08