A way to possibly tell if it's a control issue is to completely ignore the poop issue. No special adjustments such as telling her to tell you so that you can put a diaper on her. Tho that might help. When she makes a mess, clean it up without any sort of emotion. Act as if pooping in toilet is not important to you.
I think that telling mama that she needs to poop and doing it in her diaper could be a developmental issue or even a security issue. She/he may feel more secure with a diaper on. She may poop because she relaxes knowing that there is a diaper on. She may have learned how to hold it but not how to let it go.
One reason that some toddlers/preschoolers won't use a toilet is that they're afraid that part of them is being flushed away. This would seem to me to possibly be a fear because poop has shape and substance and it came out of their body.
Waving "bye bye" along with an explanation on their level that it's good for poop to go bye bye. This won't work for some kids. Using a potty chair could help with this too if Mom doesn't put the poop in the toilet and flush it in the child's presence.
How long has your daughter been using the toilet for pee? Being able to poop in the toilet comes much later for some children.
If this were happening with me and my child I'd try not paying any attention to when and where she poops for a few weeks. I think, that if you don't make a big deal of pooping she will be trained by August. I wonder if you've connected going to preschool with pooping in the toilet? That would be a natural way to try to encourage her. However, she may be anxious about going and thus not want to poop in the toilet. Her anxiety may not be obvious. She may even be enthusiastiv about going at the same time she is anxious about it.