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It sounds as if your plants are infected with Rust.
Neem Oil is a good treatment.

https://www.gardeningknowhow.com/plant-problems/disease/learn-about-plant-rust-disease-and-rust-treatment.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

If you feel the grass needs some rejuvenation you can cut back to about 8 inches in height.

Otherwise just leave the grass for winter interest.

https://plants.usda.gov/factsheet/pdf/fs_ange.pdf

It would be best to plant up in pots until you can place them in the ground.

Leaving them out or in water will likely kill the grasses.

The care will depend on your growing zone and the hardiness of the perennial grass that you have planted.
Potted grasses can generally be moved to a shelter location, such as a garage or shed.
They can be wrapped with winter protection blankets to keep the roots from freezing.

https://www.gardeningknowhow.com/ornamental/foliage/ornamental-grass/ornamental-grass-in-pots.htm

The old grass from the previous growing season dies off and is replaced by new grass in the current growing season.
In the early spring, after the threat of frost, you can trim the entire plant back.
Keep the plant level to about 3 or 4 inches cutting the length uniform to keep the nice domed shape.
Keep the plant combed out of old grass or damaged foliage through out the season by combing out the grass with your hands.

https://www.gardeningknowhow.com/ornamental/foliage/blue-fescue-grass/blue-fescue-ornamental-grass.htm

You actually didn't "have" to cut it back in the fall, but it shouldn't have hurt it. Depending on the zone you're in, how much sun your Porcupine Grass is getting and the temperatures you've had so far this year, it could simply be getting a late start. Keep in mind that although Porcupine Grass is not at all fussy about soil content, it does need plenty of water. This article may help you: https://www.gardeningknowhow.com/ornamental/foliage/maiden-grass/growing-porcupine-maiden-grass.htm