I can't find two base game hairstyles neither in my game nor in body shop: the long straight hair (Dina Caliente) for female sims, and the very very short hair (Oberon Summerdream) for male sims. The hairstyles show up for these sims, but if I change them, I can't change them back. I don't have mods that hide them, to my knowledge (the hiders I have shouldn't do anything to these)
Does anyonw know why this could be happening?

This is way out of my field of expertise ( who we kidding, I have none lol) , but the only things I can think of are the hiders you mention or default replacement hairs. Neither of which I'm familiar at all with lol, but have you recently added any hair hiders or default hairs? That's all I've got, sorry. I'm sure more knowledgeable peeps will come by and help. I'm sure you know the drill for running a test game/folder with no cc, have you tried that ?

I'm an idiot, I posted before testing. It was the hider to hide the M&G dreadlocks and only leave one that was bugging out. Not sure what to do about this thread now. Do I delete or leave for if someone else has similar problems?

In my opinion, just edit it to change the title by adding "- SOLVED" to it, and leave it for future reference.