Recently I came into a weird situation. So, to start with ...
I got Sitecore 9.0 update 2, installed SXA 1.8 for 9.0 on top of it. Created a tenant and a site, also made few pages for test and some initial settings as we usually do as per documentation, so far so good.
Then I made a solution, configuration and serialized everything I made on the previous step, then reinstalled an instance in order to test my solution, configuration and serialization deploys well on top of clean Sitecore and to confirm everything works as expected.
Build process went fine, deployment worked well, Unicorn ran and created the site with all my pages and all relevant items - all good, except for one issue. Before Sitecore reinstallation, a new page in Experience Editor looked that way:
After the deployment on top of the new clean instance, running Experience Editor brought the following layout:
Clearly, there was something missing from either my configuration or serialization. So I started troubleshooting and comparing.
In both scenarios, I had instances sites of the same version and both with SXA 1.8. Since this issue was reproducible, next time I made git init just within webroot in order to get a version diff with a post-deployment state, so that I could see all modified files and see the diff as I usually do with git tools. However, nothing suspicious found at bin and within App_config folder. Deployment went as expected so that should be something in Sitecore then.
First of all, I compared templates and standard values, but they were the same for both cases. Next, I ensured presentation details are also identical, even ensured Layout files and binary equal. Yes, it contains body-top placeholder, but in one scenario it is presented for some reason.
Then I went to the actual page items in order to compare those, but they were also the same. View options have same parameters checked. No idea then...
Finally, after spending plenty of time I figured out that something was wrong with SXA themes. Unfortunately, did not identify what exactly causes the problem, but after re-creating it at serialization config and re-serializing items the issue has gone. So, the outcome is: if you see unexpected placeholders - please make sure your SXA theme is in the correct state and serialized properly.