It’s easier than you think
At a guess, it will come down to police not swapping rubber gloves. The procedure would be when an item is located you change gloves to ensure they are sterile so whenever the item is picked up they can’t be an acquisition of transfer of DNA.
The raid would of been video tape and when they were searching they wouldn’t have realised they were picking up a kg of MDMA (it could of been in normal packaging).
When they realised what what they had it would of been too late because it was video. Realising the money that will be thrown at Thompson defence it wasn’t worth using that evidence.
The cladestine unit also deals with drug labs(the only unit that is allowed to make them safe) and are required to make the lab ‘is safe’ before a complete search is completed. In the process of making the lab safe the above could of occurred.