i got the balduran's giantslayer just a bit ago and tried it out of curiosity and the giant form cancelled the elixir of colossus (it may work applied the other way around, i'll check later)
i tried using enlarge spell on top of that to see what happens and it ended up cancelling the giant form, including the temporary hitpoints (completely swapping the effects)
i checked the wiki before and remember it specifically saying the sword and the elixir mix, it'd be a shame if this is an intentional patch since it somewhat hinders on creativity - or at the very least some sort of popup saying it'll cancel another effect