They can still get an overgrowth of coccidiosis even on medicated chick feed. The Amprolium in the feed is the same as the med in Corid, but it is at a very low dose. The goal is to let the chick slowly develop natural immunity to the coccidia as the medicated feed only retards the growth rather than eradicating entirely. The idea is to let the chick grow until it has that immunity, then stop the medicated feed (generally around 12 to 18 weeks) so that as an adult it keeps the protozoa at bay naturally with its own immunity as the coccidia are ever present in the soil.
However, if there has been an overgrowth in the environment (moist, warm, fecal invested....your typical brooder), then the chick can be overwhelmed by the coccidia creating substantial overgrowth that has to be corrected with a larger dose of either the Amprolium (Corid) or another drug such as the sulphur type in Sulmet.
Pull the medicated feed while you are treating as they don't need both. Return the medicated feed after treatment.
That is assuming coccidiosis....I only suggest that as it is the most common condition in growing chicks that presents with runny, smelly, diarrhea.
The other is simple bacterial overgrowth from eColi or pasturella.
Yeast overgrowth can also cause really smelly, runny poo. So can worms.
There are other possibilities, and without a fecal culture, it will be trial and error. The only way to know for certain is to take a sample to a vet for culture and then subsequent diagnosis and treatment....otherwise, you begin with the most common cause of those kinds of symptoms in light of the history and begin trying treatments.
That's why I like the Sulmet, it is broader based than Corid and can treat bacterial as well as the protozoa in coccidiosis.
The ACV would have nothing to do with the change in bowl habit. It might have caused a slight case of the runs, but not profuse diarrhea that smells bad. That is something amiss.
LofMc