I was thinking maybe your partner's Tamagotchi has different programming or maybe a glitch? I was thinking the former because I looked up the red glitter Tamagotchi on Bandai's website it says has the original programming, but looking further the page for the 20th anniversary says the rereleases have the same programming as the 1997 Tamagotchi's which we know isn't true, so I'm thinking maybe it's a glitch.
The 20th anniversary tamas vulnerability to snacks reminds me a lot of the Ocean, as if it has too many snacks it will die when it's supposed to evolve. Also, I believe for most vintage tamas 3 sicknesses in 1 growth stage = death.