Google is ignoring canonical tag. Should I use a 301?

My domain's home page used to appear in Google's serps for my site's main keyword. The page that is displayed as the home page has it's own url too (had to be created as a page by the CMS). So both pages are identical but contain the canonical tag informing google that the homepage is the source. Recently Google decided to start showing the internal page in the serps and is basically ignoring the canonical tag. Site has dropped several places in the March 2019 update and I suspect this is the reason!

1) Has anything like that happened to any of you before? if so, what did you do?
2) Since the internal page that is now being displayed in the serps only exists due the CMS, would it be a good idea to do a 301 to the homepage?

I also have similar problems with my Shopify website. In my case the audit report says there are some non-canonical URLs (canonicalized) in the XML sitemap. So google got confused which pages should be indexed. This can result in search engines ignoring your sitemaps. I have very little knowledge in SEO and I also want to know how to fix the XML sitemap.