Okay thank you guys, these parts were < 0.4 mm that's why they wouldn't show on Cura after being sliced.
I've ticked the "print thin walls" box and indeed the thinner parts are now showing regardless of the scaling I use but I'm facing another issue now: when it's supposed to print the thinner parts, it does this (https://streamable.com/txk2o) instead and doesn't print at all (the filament keeps going back and forth and therefore the cold end (stepper motor) doesn't feed the filament to the nozzle).
I mean, I've had this stepper motor issue for about a week now, but I thought it wouldn't be a big deal, as it wouldn't occur everytime the printer was functioning. I was wrong.