Can't pages of subsites have similar slug?

They can.

Are you 100% sure these are subsites with their own, separate, admin area?

Yes. I am positive.

*With one of the subsites, it was my fault. (I somehow didn’t notice that slug was used).

*With the main site, however, it is puzzling.
It seems that: there are “hidden” pages with the slug “home” and “home-2”.

I might have created such pages months ago. But they are long deleted and removed from the trash.

In fact, I don’t even see the page I am using for “static page”, though it is working.

Maybe it’s a tag or something, try to access: https://example.com/site-1/home/ and see what type of webpage are you dealing with.

*With the main site, however, it is puzzling.
It seems that: there are “hidden” pages with the slug “home” and “home-2”.

Can you elaborate on what you mean by ‘hidden’? You mean the URLs still work but there are no such pages?

Sorry for my belated reply.
I wanted to see the impact of some changes first. The problem is not solved yet, but no longer bothers me much.
However, in case WP pros are interested – esp. to those who commented above -, let me clarify the problem.

@alin Marcu
As I corrected myself in the second post above, the problem appears on the main site only. So, I conducted your recommended test on the main site.
When I try to access: https://example.com/home/ and https://example.com/home-2/ ; I find a page that doesn’t load fully.
It displays the header and the top navigation menu and -immediately after it – the copyright notice (which is normally at the bottom of the page.)
[By the way, there is no category or tag with the slug “home”.]

@ Ipstenu (Mika Epstein)
I was talking about two types of “hidden pages” issue.
Type-1/Even if I deleted the static page from the main site’s post’s page.( /wp-admin/edit.php?post_type=page). The main site was working as if the page exists.
That is no longer the case now. (If I delete the page, it will immediately go to the default homepage appearance).
Type-2/The two pages I mentioned above https://example.com/home/ and https://example.com/home-2/ appear to exist somewhere – hidden from me.
Their slug is still unavailable for new pages. When you try to access them, you don’t get 404 rather a strange page. [that displays only the header and the top navigation menu and -immediately after it – the copyright notice. The rest of the page blank)

Note:
The problem is no longer bothering me; b/c readers can no longer see the “home-1” slug when they access the site. (Initially, I was using WPtouch mobile plugin and the mobile home page was showing the slug.)

After I posted the question above, I upgraded the theme into its premium version, deactivated jetpack mobile and removed WPtouch mobile plugin.
The first type of “hidden page” issue might have disappeared as a result of those changes.

But the rest of the problem (the two “hidden” pages) remains.

Thank you.

Today seems to be the day of people come back after months ….

Did you empty the trash on your site?

?? Right. Sorry, I apologized for the delay.

There was one factor I didn’t consider.
I use “WP Mu Sitewide tags” plugin and it imported pages [to the main site’s pages’ list.( /wp-admin/edit.php?post_type=page) ].

Interestingly, the slugs of the imported pages is different from their original slug.
For example: https://example.com/site-1/home/ is imported with a slug “home-2”. Yet, it directs to the same home page.
The same goes for pages imported from the other subsites.

Therefore, my guess is that the main site is considering the imported posts as its own. And, refusing to use similar slugs.
I wonder if the same is happening with imported blog posts too.
=======

So…..I deselected the “include pages” option from “WP Mu Sitewide tags” plugin settings.
And, I intend to delete the imported pages, as I see no point in importing them (wrong?).

P.s. – for now, the site is refusing to implement the edit, delete, etc commands on the old pages. I don’t know why. I will try again later.

That would make sense. Duplicate pages and all. I wouldn’t import them either to be honest.