Your hvac system can sweat. I was getting a little mold on my filter. I had them put in a black light and haven’t had any mold since. I have quite a few friends that put this light in their hvac units to get rid of any mold

I agree with TommyT. Make sure your doors to the air handler closet are sealing. Turn on lights outside the door, go into the air handler closet and close the doors. Look around the edges of the doors for light. Where there's light, there's air getting in and needs to be sealed.

This. Common. I have worked on HVAC in the past. However, you can verify by placing a strip of paper around the unit about 2" from the bottom with a magic marker line drawn on it. Once the ink gets wet it will smear. If you get nothing on the paper, then look to the inside. The staining on your filter leads me to the inside.

You can remove the filter and get back into the box with a flashlight and look up inside. Condensation can occur just like on a window sill, but this on an ac unit seems really odd. Either you got major humidity issues in the house coming in by the unit which I think you would see all day long or it's the common drain issue.

If it's come from the inside, someone needs to take the cover off and look at the pan and coil. The pan could be cracked, misaligned, etc. Also you have to remember the air is rushing by the coil inside there at ~1200 CFM, some units have shields to direct the water into the pan.
Needs to be fixed, you will have a rotten wood in no time.

Having air handlers in the garage in a hot and humid environment cannot be good for these units. I would have to believe that they do not last as long as those air handlers in the house.

My air handler is in the garage and although you want to make sure your doors are closed, you are not supposed to get condensation like that outside the unit. You either have a cracked pan, partially clogged condensate drain or the filter door is leaking air. Those doors are terrible since they don’t make a good seal. You need to get that fixed.

Seal, Insulate and Mini-Split
 

We had a similar problem with condensation. It was so bad that water was running down the front of the unit and ended up corroding the switch on the electronic filter ($300 to replace).

Each technician that visited would add more insulation and seal the unit better. One even re-routed a wire and then plugged the hole where the wire had been. These action reduced the condensation but never eliminated it.

The ultimate fix was to install a mini-split in the garage. No condensation since.

(My guess is that the HVAC shouldn’t be in the garage in the first place.)

We had water on our filter and it was the coils leaking inside the air handler. Major job to replace the A coil. Same unit as yours, covered under waranty for parts on labor cost me around $1000

The coil doesn't have water in it. It has refrigerant. The only water is from the condensation that forms on the outside of the coil. So, replacing the coil because you have water on the filter doesn't make sense.

This won't happen down here, but up north, neighbor had water leaking around light fixture. I went up in the attic where our HVACs were located and found the return duct tube about 5' away was extremely low over that spot at the ceiling where the light was. Quickly realized the insulation was wet. The low spot sloshed with water. Went back to air handler and looked below A frame. The plenum where the returns all go to was filled with about 12" of crystal clear water and slowly overflowing down the lowest return duct tube. Took hose and blew out condensate line, sucked out water, and it seemed to stay dry according to neighbor. Probably 5 gallons collected in there. Of course had it just overflowed the pan, they would have quickly realized it.

Yours has to be an inside handler issue. IMO