Is Civi supposed to redirect after a contribution form submission?
So when filling the form, you submit it (the data is into the post, so you have the feeling it's truncated as you don't see the params in the log/net tab). Once that data is processed, it needs to display something (the confirmation that everything went well).

They are two ways of doing that:
- either the result of the post is the confirmation message
- or you redirect to another page that displays the confirmation message

the second one (the one yo experienced) has several benefits: it's easier to redirect to another page (so you have a completely custom and different thank you page from outside of civi, and as importantly, it allows you to reload the confirmation page without resubmitting the contribution (that's why you have warning from your browser when you reload a page after you submit something)

Now, that design choice made many years ago doesn't make that much sense when you have forms in ajax (eg. with payment providers like stripe), but I don't think it's a problem to do a redirect, it doesn't create a major speed difference.

So in short: it's normal behaviour, nothing to worry about ;)