I'm still new to maintaining my houseplants...so don't take my advice for definitive truth. Yours looks similar to what mine does and here is what I've concluded is the likely issue: Improper soil drainage. Many users have noted that even their local nurseries tend to use soil mixtures that are 1 part peat, 1 part perlite or some variety of that. The soil really ought to be (per Al's suggestion) 5 parts pine bark fines, 1 part spagnum peat moss, 1-2 parts perlite with garden lime. The pine bark fines help aerate the soil and allow the roots to breathe. A good article on that can be read here. Maybe it's an issue with the roots as well? Or a mixture of the roots being overcrowded & drowned in water? When the leaves get curled on a monstera, it could be a sign of low humidity. Maybe try misting the leaves more or using a humidifier?

Hi Michelle! Did you figure this out already or am I stepping in too late? The top photo is an older more mature leaf shape, the lower is a more juvenile monstera that has just begun to split. If allowed to climb and given enough light, the lower pictured plant would eventually develop more fenestrations and get that large (also given that it is actual deliciosa and not deliciosa v. borsigiana). Hope this helps!

Hi robin98(NZ(9b)) thanks for your help! *I've had the plants for approximately 2 years. *I repotted them approximately 1-2 months ago. *Both were in terracotta pots, now just the larger one is. The smaller one is in a plastic pot now, there's been no change in either plant other than getting worse. *I water both when the top 2 inches are dry, I use my finger or a wood dowel to tell if the soil is damp, mainly my finger. If the top 2 inches are dry I water until soaked, with the water draining freely through the holes. I never let them sit in water for longer than maybe 30mins, after watering. *The build up was before I repotted, it looks like salt or water hardness. I removed them from those pots and placed them in fresh pots that had been disinfected prior to use. There's some build up since that time on the rim of the terracotta pot (can be seen slightly in photo below of the top of the pot) I'm still sad, but learning to cope with it! ;) Well, I'm trying to learn from this experience. Thank-you so much! I've included some newer photos, please feel free to ask more questions if necessary. I'm considering removing the large one from it's pot, to repot again, but into a smaller pot after I remove some roots. It smells from the top of the soil now, and although when I repotted previously there was no indication of rot I'm thinking this is either leaf disease of some type or root rot. I removed these leaves: Then this started: Here's a picture of the top of the pot: Thanks everyone in advance! Let's hope we can save these guys!

Difficult to say what exactly is the cause. There are two easy to notice mistakes you did though: first, the plant is buried too deeply second, you should always water immediately after re-potting, otherwise the new, dry soil will suck the moisture from the old soil the plant was in. Either one, or both of these mistakes could be the cause, or at least contribute, to the yellowing. .