How to I eliminate the smell coming from my Mitsubishi mini split units?
I wonder if the mold is growing in a place in the drains where cleaning solution isn't reaching. I can think of different possibilities to get the cleaning solution on the entire inner surface of the drains:

(1) Add cleaning solution as directed, then use a shop vac for a short time on the end to vigorously pull cleaning solution through the drain.

(2) Add solution as directed. then plug the drain to let the solution entirely fill the drain.

(3) Make up a quart of cleaning solution in a jug and introduce this into the unit from the discharge end outside so that the solution completely fills the drain. This risks the possibility of an overflow of solution into the evaporator unit inside and so one would have to be careful.  I'd use a length of clear neoprene tubing with swollen sections so you could cut at a swelling to get an end of the right diameter to seal the discharge end of the drain. By alternately raising and lowering the jug one could flush the drain repeatedly.

(4) Attach a  6 ft or 8 ft length of neoprene tubing to the end of the outflow tube outside and raise the end of the neoprene tubing to the height of the drain pan inside the evaporator.  Introduce the cleaning solution into the drain inside as instructed. The solution should completely fill the drain system and overflow out the end of the neoprene tubing.