Ivermectin goes threw a dogs system in about 24 hours. To kill mange it takes a dose a day for up to 5 days. An thats at around 8 to 10 times the approved oral dose every day. We are talking kidney damage levels or even death in some breeds.
To kill fleas you would have to do the same every 2 weeks or never let the dog go outside.
Now you may be right that you are using something other than what I am useing. I use 1% injectable given orally. Its the only thing I have ever known any breeder or vet to use on dogs. They sell a pour on for cattle that is useless on dogs cause they do not absorb it threw the skin like cattle do. Now if you soak a dog in the pour on an leave it on, assuming the dog does not OD by licking its self it may kill the fleas by contact, but dawn dish soap does the same thing.
The only place I see online where people are even talking about using it for fleas in dogs is just people asking about it an being told no by breeders an vets. An one guy saying how great it works for fleas an ticks on his one dog....
Now if you can point me to somewhere online where a breeder or vet is talking about actual dosing of Ivermectin to treat for fleas I will gladly read it.