I don't think this is surprising. Good technical writing is very precise. If you're starting from non-technical writing, I suspect that in most cases you can't make it "sound scientific" without adding new claims or changing the meaning. (Maybe you are being more careful, but this is something that worries me in general.)
"More scientific" is not some merely stylistic thing that faithfully preserves the original meaning of what you wrote. The precise details of each paragraph matters a lot in terms of what and how it communicates. The fact that these details do matter means that, according to my accounting, it is not entirely your paper.
Also, I find striving for "scientific" to be a pretty undesirable thing. Why should papers read like that? What is the benefit? The best papers (in terms of their writing and communication) are unpretentious and conversational. I'm pretty sure I'd prefer your "basic sentences", especially if they were wholly yours. (I understand that there are also external forces at play here as you mentioned.)
But I suspect that a lot of academic's feelings about it are informed by what others have told them and how they've been trained, rather than by what's actually permissible in the publishing system.
There really is no point, as long as you verify the content matches your intent and edit out anything poorly written.
Frankly, I've read plenty of papers by native English-speakers over the years that'd strongly benefit from being rewritten by an LLM too...
This is a sin.
Here's a paper by a non native English speaker. The lack of the formal style doesn't cause any issue. Preciseness is what matters.