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Have they been getting too much water? Mine always turn yellow if I overwater or even underwater. These are great, tough plants. Give them time to recover from the shock of being divided. I propagate these by submerging cuttings into water which develop roots. I have way too many pothos now. :)

This could be overwatering or even under watering. The soil should dry out slightly before watering.

I would also inspect the plants for insects.
Neem Oil is a good treatment as it works as both an insecticide and fungicide. It's safe for people and pets.

You also can trim up the plant to clean it up. Repotting and refreshing the soil may be a good idea if this has not been done.
Salts can build up in the soil and cause leaves to turn brown.

Here are some links with more information.

https://www.gardeningknowhow.com/houseplants/pothos/pothos-plant-care.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

New leaves will generally be smaller than leaves at the base of the plant. Keeping your plant trimmed back will help the plant become bushier.

Brown spots could be water spots or even insect or disease damage.

If the spots increase or you see other signs of it spreading, treat the plant with Neem Oil. This works as both an insecticide and fungicide.
It's safe for people and pets.

Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/houseplants/pothos/pothos-plant-care.htm

Check the soil to make sure the first inch or so is drying out before watering.
Over watering can cause root rot.
Also if watering with conditioned water it can cause black edges on the plant. Try leaving the water to sit 24 hours before using for plant watering.
If the dark edges spread it could be a fungus. Neem Oil is a good treatment.
Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/houseplants/pothos/pothos-plant-care.htm

Without seeing the substance I would be on the cautious side and treat with Neem Oil.
Neem Oil works as both an insecticide and fungicide and is safe for people and pets.

If this is a newly purchased plant it is likely it is a residue from watering, fertilizer and pest control from the greenhouse grower.
It could be just wiped away with a damp, soft cloth.

Here is a link with more information.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

You can plant the cutting now that it has roots, but if it is long and leggy I would trim it back to about 6 inches in length.
You can root the section that you cut off also!

https://www.gardeningknowhow.com/houseplants/pothos/propagating-pothos.htm