Short answer is maybe… it depends on the lender and how sophisticated their policy/systems are.
They will/should be able to see that you have a cc with a balance, but that it’s on a promotional rate, they will also see that you are meeting your obligations and not missing payments.
If you are towards the max balance it will effect your utilisation ie have you maxed out your borrowing capacity does this indicate a high risk…
On balance, generally speaking, having access to and using credit, whilst meeting obligations will result in a low risk profile… so would count as good in your case…
If you are applying for more credit, it would be a good idea to have your balance a good way off the max limit however.
Then it’s a case of passing affordability etc….
Clear, no? That’s kind of the point really, companies like to keep their policy as a bit of dark arts to stop them being abused, so, generalities only…