Google Detecting Real Page as Soft 404 Error

[bheard]

We've migrated my site from HTTP to HTTPS protocols in Sep 2017 but I noticed after migration soft 404 granularly increasing.

Example of soft 404 page: https://bit.ly/2xBjy4J

But these soft 404 error pages are real pages but Google still detects them as soft 404. When I checked the Google cache it shows me the cache but with HTTP page.

We've tried all possible solutions but unable to figure out why Google is still indexing to HTTP pages and detecting HTTPS pages as soft 404 error.

Can someone please suggest a solution or possible cause for this issue or anyone same issue like this in past.