Applying transparency to a patch that's exactly 320x200 in size will instead fill the screen with a solid color
From my initial description: https://mb.srb2.org/threads/applying-transparency-to-a-patch-thats-exactly-320x200-in-size-will-instead-fill-the-screen-with-a-solid-color.33496/
@MonsterIestyn previously mentioned this is due to the F1 menu being 320x200 in size. Prior to Lua, this wasn't of any concern. With the introduction of Lua, and custom patches, this can be a limitation for those who want to make custom patches that are exactly 320x200 in size and want to apply custom transparency.