Confluence Embedded Pages (CEP) allow you to publish Confluence content in external systems. It's powerful stuff and here's some examples.
If however you are using apps and macros from Atlassian Marketplace, it seems that some implementations are better than others.
In the example below, we are using Table from CSV from the Atlassian Marketplace. It is a macro built on Atlassian Connect. Here's what it looks like in a Confluence page:

It displays tabulated data in your page.
To aid things visually, I've added
"👇 👇 👇 👇 👇 Macro here 👇 👇 👇 👇 👇 " above the macro, and
" ☝️ ☝️ ☝️ ☝️ ☝️ Macro here ☝️ ☝️ ☝️ ☝️ ☝️" below the macro.
If the macro is not displayed, you'll just see these next to each other
Confluence Embedded Pages done right
The following examples display the macro correctly
Refined for Confluence
Everything works good in Refined's use of CEP. You can see the table of data.

Jira Project Pages
Everything works good in Jira Project Pages' use of CEP. You can see the table of data.

Confluence Embedded Pages done wrong
The following examples do not display the macro at all.
Confluence Cloud for Microsoft Teams (by Atlassian)
I couldn't get this integration to work at all in the native Microsoft Teams app, but using Teams in the browser, the page was displayed.
Unfortunately, the macro was not displayed correctly, and worse there was not even a placeholder to show that content was missing.

Miro
Miro's implementation of CEP was the same as Atlassian's Teams integration. The macro was missing without even a placeholder:

I've tried to understand what's different for the Teams & Miro versions of CEP that stops them from working properly, but have not worked it out yet.
Perhaps it's something to do with iframe sandboxing or the content security policy of some frame ancestors.
If anyone knows how to fix these so that they work correctly, I am all ears.
I'm also happy to update this post as and when each is fixed.
Cc: @Lennon Liao @Dugald Morrow