I have the same issue. From yesterday, I have started a task of website audit. In first stage, I audited our entire blog posts one by one. I made a checklist with some important fields like Title, Description, Broken Links, Mobile Friendliness, Google Indexing, Load time, and much more.

When I tested our latest blog post on Google indexing, it showed that it was not indexed yet. The last blog post was indexed on 7th September and after this date we published around 5 blogs and none of them are indexed on Google yet.

However, when I inspected the URL on Google Search Console, it gave me an error HTTPS is invalid and might prevent it from being indexed, which might be the cause of why it wasn’t indexed.

I was thinking that maybe it happened due to SSL expiry. For this, I also discussed this matter with our IT head but he said that this is not an SSL issue.

I read several Google guides and tips and have followed their steps to identify the error but I didn't find any such error in our website or in source code.

Anyone can help in this because this is a critical issue.
Thanks in advance