Why won’t my mulberry tree produce fruit?
The tree gets male blossoms by the hundreds, fruit sets, the fruit gets about a quarter inch long, and falls off.
Mulberry trees can be a puzzle. Some are dioecious, meaning there are separate male and female
plants. Male trees flower but don't fruit, so I don't think your tree is male. However, if it were a female that wasn't getting pollinated, it wouldn't be making fruit at all, either. There are also many mulberries that are self-pollinating. And there are also mulberries that are fruitless. I wonder if yours is sort of caught in a mulberry netherworld of not being completely any of those things.
This article talks about the care of fruitless mulberries, which should be pretty close to what is required by other kinds of mulberry. https://www.gardeningknowhow.com/ornamental/trees/fruitless-mulberry/fruitless-mulberry-trees.htm
You could think about changing the light, water, or fertilization, to see if it helps the fruiting.