We are aware of this, it seems to be isolated to iPad running iOS 12.x.x. Users who have upgraded to 13.x.x have cleared the issue.
We have reported it to our advert provider and they are investigating. It is not easily reproducible though as some people with exactly the same configuration (myself for example) do not have the problem.
If you have the problem and it is not on iPad running iOS 12.x.x then please do raise a support ticket with as much information as possible and we will look into it further. If it is then please either try upgrading your iOS to 13.x.x if you can or bear with us as we are trying to resolve it.