I think the body does lose weight when it dies. I looked it up after watching a Sean Penn movie a long time ago. In some experiment, a (hopefully) willing person sat on a scale. Then died. The scientists repeated that however many times necessary to fulfill the ritual and the results were that a certain amount of kilograms mysteriously go away when humans die. I explained it away with the evaporation of sweat but the stuff I read disagreed so I have been left to believe it since. They could have figured that one out by now but it was a thing at some point in time.
(emphasis added)
Not "as many times as necessary", but 6 times, which is not a large enough sample size by any measure. And of the six times, two were discarded as of no value (one of which because the subject died before the measuring equipment was set up), while others showed fluctuations in weight after death - falling, rising, falling again to settle. The oft-cited result of 21 grams was actually only measured in one instance. The experiments were never repeated by the original experimenter or by anybody else, so no independent confirmation can be shown. Sweat evaporation cannot be ruled out, as similar experiments with dogs - which cool off by panting instead of perspiring - showed no weight loss on death. And finally, the experimenter, in lieu of considering other possibilities, just said that the results led him to the "inescapable conclusion" that this was the soul escaping from the body - a conclusion that just so happened to coincide with his religious beliefs.
So no, these 1907 experiments shouldn't have "left (you) to believe it since". They're early 20th-century pseudoscience and, if they haven't been as thoroughly debunked as they might have been, it's only because nobody took them seriously enough to bother doing so, at least until the film "21 grams" came out.