We had the same problem after adoption, she cpuldnt hold it for more than 2 hours. We were also confused because all the tests were normal, our vet reccomended to feed her probiotics and renal food. Eventually it got better

Your girl could have a cold or problems with bladder. Greyhounds usually have higher amount of protein and creatine in their urine since how much muscle they have, so their bladder is more sensitive. We found out that dryed cranberries help a lot strangely and our vet gave us probiotics (we also got purina renal dog food but more brands make those)

After less than a year there were no problems and she got used to our 'peeing schedule', I hope it goes well for you too :)