I’ve checked your Site Health information and I don’t see anything unusual. I’ve also looked into this query further, despite this being a Search Console question. From what I’ve gathered not all AMP URLs may be listed under the “Enhanced > AMP” tab within Search Console, in particular after some changes were made earlier this year, which results in coverage of a smaller number of pages.
The below contains more information:
https://support.google.com/webmasters/answer/6211453?hl=en#amp_enhancement
So while there may be less AMP URLs being reported on within Search Console this doesn’t mean they are not indexed as AMP URLs.
Note I also have an AMP standard more site and I can see a similar decrease. When I check the “Coverage” tab it lists more AMP valid URLs than I see in the AMP tab. What I did then was check an URL which is not listed in my “AMP” report but which is listed in the “Coverage” report. I performed a search including my site URL (Examples: site:example.com keywords or slug) and indeed I discovered the site in Google search, despite that specific URL not being listed in the AMP valid URLs report.
From your side you can find any older AMP URLs isn’t listed in the “AMP” tab under Enhancements you can always perform a mobile search to ensure it’s discovered. You’ll notice that while it’s not listed in the valid AMP URLs tab it is indexed.