Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
it isn't any surprise that the issue started AFTER the fluid change IF it was flushed. It's fairly common for the flushing and solvent to actually cause minor seal leaks once all the debris and clutch material is flushed out. If a transmission is not flushed regularly then I never recommend flushing just a fluid exchange.
Resolving the issue at this point is not going to be achieved very easily. You likely may be able to drive like this for awhile before the transmission gets worse, but repairing it will involve opening the transmission and their really are no cheap easy options once you get to that point to be honest.