Deleted messages

Not really sure if I’m overthinking this or not. My partner and I recently got upgrades on our phones so I’m not sure if this is because of this or he is deleting messages. I went in his WhatsApp yesterday evening to send myself some pictures and see what ones he’d send to his mum. For background, my partner lived in Australia for 4 years and whilst he lived there he had a very close female friend that he lived with on and off, had sex with when they first met but swears he didn’t do anything in the years after that they were friends and that their relationship is entirely platonic. She messages him now and again saying she misses him and she’s told him she loves him before (I think in a friendly way). Their relationship has always made me uncomfortable. Whenever I went on his phone and I always saw their chat in WhatsApp and it was fine recently, however it’s no longer there? I’m not sure if he’s deleted it and if so what on earth could he be hiding. It makes no sense that it’s not there because he hasn’t deleted other old chats that have transferred over to his new phone and I can’t figure out why on Earth he’d decided to delete their chat over various other chats that are really old. Should I confront him and ask or should I just trust that it’s an innocent reason? I haven’t wanted to say anything as I don’t want to ruin Christmas with an argument.