I believe this is fairly common. There are subtle differences between older tests and new tests, so it takes a little while to become adjusted to the new material. As long as you keep practicing with it, and learning what to expect from it, you should be just fine with the newer tests. If you're taking in June you should try to only study with more recent material, if you can.

I think you should remember that your goal in taking PTs isn't to accurately predict your score, it's to get better at the LSAT. So, yes, you will remember some material that you see, but I hardly think you'll have all of the answers memorized. There is absolutely value in going over old material and applying what you have learned since you first saw it.

What test #s are you using? I experienced the same thing when I switched from older tests to more modern tests, and I think it's fairly common.