`entries_layout: grid` broken in various layouts

[mmistakes]

https://github.com/TarodBOFH/tarodbofh.github.io/tree/mm-bug-with-grid-layout

no entries_layout: grid defined on frontmatter == posts and categories broken and home list as expected
entries_layout: grid on frontmatter == posts and categories broken bug home is working

Originally posted by @TarodBOFH in #2616 (comment)

[TarodBOFH]

The problem comes with {% assign page = post | default: page %} on the post__meta.html file. That overrides current page variables with those in the post, and effectively removes any variable set at page lvl. I don't know if there is a way to combine objects on liquid.

I'm no liquid expert with that override can be dangerous. Is there a way to make it behave differently? I.e.

{% assign frontmatter = post | default: page %}

And use that local variable instead of the global one. I'd think to offer something that combines post and page in a local variable instead and make that avaliable to the includes, to avoid problems like this in the future.