This is not a bug, but rather a problem of the materials you picked to summon Borrelsword Dragon.
While it’s true Knightmare Unicorn can be treated as either 1 or 3 materials for a Link Summon and Borrelsword requires “3 or more Effect Monsters” :
You can only use a Link Monster as 1 material, or its Link Rating worth of materials (3 in the case of Unicorn), while also respecting any other requirement, and matching exactly the Link Rating of the monster you intend on summoning.
In your particular case you must have : 3 or more Effect Monsters, while also matching Borrelsword’s rating of 4.
Long story short : You cannot use a Link-3 as 3 materials for Borrelsword and respect Borrelsword summon requirement of 3+ Effect Monsters. In your particular scenario, Unicorn + Linkuriboh + Orange Light can only make a Link-3 or a Link-5. You would have to add a fourth monster, and treat Unicorn as 1 material.
If this explanation isn’t clear, feel free to ask for clarification.