If you had a very high oil level and had also replaced the boots, then oil would indeed be able to leak out at the wheel. As I remember, a minor amount of oil is supposed to be in the boot to lubricate the sliding gear reduction unit AND one of the rear wheel bearings. (The other is lubricated by injected grease.)
Oil seals and o-rings will harden and fail to seal after 10 years or so. Therefore, that seal probably should have been replaced during the boot swap. The wheel seal wasn't leaking previously because the leaking boot (which you replaced) never allowed oil to get over that far, AND/OR the oil level had never been that high.
IMHO everything points back to a simple case of chain case over-filling. With the mower set on its rear end, mower deck pointed to ward the sky, you'll see 2 "pop-in" plastic plugs on the chain box. Pop out the plastic plug furthest from the floor (the highest position), and fill the chain case to that hole. I prefer 75/90W gear oil because it moves more slowly. Then push the plastic plug back in the hole. The chain will dip into the oil puddle and take it to higher positions. You don't need a flood of oil inside the axle boots.
Hope this helps.
PS. Any comments on: 1. how you filled the chain case after boot replacement, and 2. what oil you used ?