Skinny duck, barely walking (Eating & Drinking normal)

[KikiDeAnime]

One of our runner ducks lost weight over the past couple weeks but she's been eating and drinking completely fine. She even becomes a glutton when it comes to food. I've checked her crop and it's full at night. For the past week, she's been inside the house so I can monitor her.
I feed the flock Scratch & Peck Grower mash, since the ducks live with the chickens, and I've been feeding it for half a year with no issues.
Her stool is normal.
I'm currently uploading videos of her that will be posted below once finished.
She's acting normal aside from barely walking and the loss of weight.

 

[Yardmom]

Eating a lot and loosing wait can be a sign of parisite. How are the chickens doing overall?
Does the duck live with the chickens all the time? Are the birds confined or free range?

 

[KikiDeAnime]

Eating a lot and loosing wait can be a sign of parisite. How are the chickens doing overall?
Does the duck live with the chickens all the time? Are the birds confined or free range?

Everyone else is doing better than she is. Completely fine.
Yes the ducks live with them all the time.
They free range everyday under my supervision(for the most part)