Flowering Lumberknot can't attack or block unless it's paired with a creature with soulbond.
If the creature Flowering Lumberknot is paired with loses soulbond, Flowering Lumberknot will remain paired but won’t be able to attack or block.
Whether Flowering Lumberknot is paired is checked only when attackers or blockers are declared. After that point, Flowering Lumberknot becoming unpaired won’t cause it to stop attacking or blocking.