When pistoning a slime block and another adjacent block in the same direction, using sticky pistons, there is about a 1/3 chance that the slime block does not retract. In the pictures, the adjacent block is an upwards facing piston, but it can be any block, opaque or transparent, as long as the adjacent block can be moved by a piston. You should be able to recreate the bug with the mechanism on the left, the pistons on the right are what I tested to make sure this was caused by the adjacent block.