i don’t think this is a bug exactly. i don’t think the system takes into account the time zone offset in your photo metadata or what you enter in that box that you’re pointing to in your Edit screen screenshot. i think it only cares about what time zone you have set in your account settings (or maybe whatever time zone has been associated with that particular observation, which in this case is London, which uses BST right now), and it assumes that any time you input is for that particular time zone. so if you want to always use GMT for input, you might try setting up a different default time zone in your account settings (one that doesn’t change over to BST). the downside there might be that you wouldn’t see dates displayed in BST. (they would show GMT instead, even when BST is in effect.)
it’s a little odd, but i think that’s the way it goes. you can read more here: https://forum.inaturalist.org/t/automatic-time-zones/7760/9.