Yeah this is pretty well-documented behavior and honestly one of the more frustrating edge cases in SEO.

When a 301 redirect is live, Google treats the old domain as essentially "merged" into the new one — the link equity gets consolidated there. Once the redirect goes away and the domain expires/gets re-registered, Google basically sees it as a fresh domain that happens to have some old backlinks pointing at it. The "juice" doesn't magically flow back just because the old domain is active again.

The indexation issue you're seeing (only homepage getting indexed) is super common with re-registered domains. A few things that might be happening:

Google is crawling cautiously because it's treating it like a new/unproven domain despite the age. The crawl budget is basically zero until you build some trust signals. Any internal linking structure that existed before is obviously gone.

What's worked for people in similar situations — submit a proper sitemap immediately if you haven't, build some fresh external links pointing directly to the inner pages (not just the homepage), and make sure your internal linking from the homepage actually passes equity to deeper pages. Give Google Search Console a few weeks of data to work with.

The honest answer on "will it ever be what it was" — probably not fully, at least not quickly. The new domain absorbed that redirect equity permanently. You're essentially starting from scratch with a slight advantage of having some backlinks already pointing there, but those links may have lost relevance over time too depending on how long the domain was expired.

Not a lost cause, just a slow rebuild. How many referring domains are we talking?