Thanks for the reply @hrishikesh! However, the binary data is not a string, so comparing the sent and received data interpreted as UTF-8 may be misleading, since most of the bytes in the data are not actually UTF-8, apart from the domain name.
In your reprod site (https://quirky-mirzakhani-33e90a.netlify.app/), we can actually see that the Content-Length being sent and received is different, and the data receieved is shorter than what was sent by two bytes:
Assuming that the reflect function is largely the same, I am still not sure why the last two bytes will get lost when received by the function handler, especially when the function does nothing but return what was sent.
P.S. I note that for some reason the response is being returned with a Content-Type of image/x-icon, which was not set by the function handler in the first place.