Hmmm... That is interesting. I haven't trained enough people with knee replacements to know if this is a normal consequence of knee replacement.
It does. Fortunately, it is probably easily correctable. Get two pieces of scrap plywood, one at 1/2" thick and one at 3/4" thick. Cut them into 10"x12" squares. These are shims that you can put under the shorter leg to even you out. Fairly common thing, actually.
The normal stuff. Be careful. Work toward full ROM before increasing load. Squat with good form. I recommend finding a Starting Strength Coach, too. Have them help you along the way. This is something where another pair of eyes coupled with useful cues will help you.