Who do you have military access with? If you're at war with the Ottomans and have military access through Serbia, then that automatically gives the Ottomans military access through Serbia as well, without them having a treaty. That could have allowed them to go say from Serbia, to Slavonia, to Bekes, and then back to Slavonia
I have no military access through either Bosnia or Serbia, and neither do the Ottomans. Posted a screenshot of their diplo screen, can upload one of mine too if it is pertinent.