That's really interesting behaviour dollyjo .
Not sure if this might explain it.
In 2009, Bristol University published a paper showing that social status in dogs is in constant flux, it’s fluid, what one dog will find of value another will quite readily give it up as he finds something else more valuable instead, for example:
A resident Labrador carries a stick everywhere he goes when out for a walk, a rescue cross breed is introduced into the household of the Labrador. The first few times they go for a walk together the cross breed dog tries to take the stick away from the Lab. With absolutely no aggression or conflict, just using body language and calming signals the cross breed dog soon learns to not bother even trying to get the stick. He has just learnt the Lab values carrying the stick above anything else at that moment.
They are then walked to a large secure field where off lead play time ensues. The Lab then will drop the stick for the cross breed to play fetch with the owner. Why I hear you ask? Because at the field they meet up with friends and their dogs and the Lab loves to play fight with his friend the chocolate Labrador, he values the play fight more than carrying the stick. The cross breed dog values the game of fetch with the owner more than play fighting with other dogs although he does if the mood takes him, usually when there’s no stick to play fetch with.
When the leads get clipped on to go home the cross breed dog relinquishes the stick for the Lab to carry home as he knows the Lab values the stick for carrying and the cross breed dog values the walk home with the owner more than carrying a stick.
So, that might indicate Tilly recognised Bella liked to carry a stick and so didn't challenge her for it because she valued the walk more than the stick. Now without Bella she can have both.
Just a thought
I'm sorry for your loss, dogs leave a hole in your heart and life x