It could be possible that they created a bot to abuse their service as you have said that's why it is nearly impossible to use their free transaction accelerator. I said that it is nearly impossible since I don't know if they really stop giving free tx accelerator or all the free tx accelerator are used right away since you know that the mempool this time is congested. When I visited viabtc, I always saw it's already in 0 free submission. If it's a visual bug then I don't know the reason behind it but I haven't heard or notice about viabtc having a visual bug on their website.