Grove

After hours of researching, I think I found the problem. PKHeX version 23.01.23 or later seems to have broke Pokédex records editor functionality for LGPE. I rolled back to version 22.12.18 and was able to change the recorded height & weight in the Pokédex correctly. With version 23.01.23 or later, you can't edit the pokémon's height other than 0 (which will result in the Pokédex having XS size) or 1 (which will result in the Pokédex recording normal size). Any number other than 0 or 1 will not change the pokémon's recorded height. And Used checkmarks are always checked (as seen on the screenshot from my previous post) for ALL pokémon regardless of whether you've caught those pokémon or not. Weight works as normal though.

So I caught an L size Snorlax, went to edit it on PKHeX (change IVs, nature), set CP to auto, got legal check mark. All seems fine when in-game too until I saw my Pokédex entry on Snorlax showing blanks on both their height & weight. Went to Pokédex editor to edit its height & weight to show the original value again but I couldn't understand the options. If someone could point me in the right direction, I'd greatly appreciate it