(Edit: I did a very poor job of explaining the issue in my initial message, so allow me to edit it with the second explanation I made in the comments.
Sorry for the mess.)
Tridents sometimes can get left inert floating in the air when pushed by slime blocks.
2 cases of the issue are shown here.
In the first one, a trident is shot straight at a concrete block behind which sit a slime block and a sticky piston.
When the piston is activated, it pushes the slime block and the concrete block and then retrieves both, which should cause the trident to fall down.
But instead of falling down, the trident stays there floating in the air, inert, physically attached to nothing.
It seems it remains "latched" onto the concrete block even though it is not touching it anymore after the retraction, as breaking the concrete makes the trident fall down.
In the second case, a trident is shot at an angle, its head landing close to the point where a slime block touches a glazed terracotta block on its side.
When pushed by a sticky piston, the neighboring slime block catches the trident but instead of launching it, it simply pushes it as it would push a block, and then leaves it floating mid-air as in the case above.
Breaking the terracotta block allows the trident to fall down - although, again, both are not physically attached to each other.