I can't remember if this has been a longstanding issue, and I don't know if it's the case for ALL pre-pubs or not. I could only find one brief mention of it on the forums (and that was for bundles, not individual books). But I do not find a "See Inside" feature for any of the dozens of new pre-pubs that are currently on the website. I've spot checked about 10-15 of them, and none of them have it. I've looked back at some older ones, like this one which I ordered in 2017, and they don't seem to have it either.
And yet, you can hop over to Amazon and see a preview of the inside of all of these books. Here's a very recent one for example. The Dark Side of Discipleship: Why and How the New Testament Encourages Christians to Deal with the Devil. No "See Inside" on logos.com, but you can "Look Inside" on Amazon.
I shouldn't have to go over to a competitor's website to help me make a decision about whether to order a pre-pub or not, especially since that competitor has the book itself available right now, which might be very tempting in some cases. I'm willing to wait for books, but really would appreciate being able to see at the very least the Tables of Contents before ordering.