I quitted to search a solution for this, actually I prefer Odoo 11, but if someone answers this question could help a lot of people, even if I find something I'll to post the answer.

meanwhile, I'm still with this issue on my Odoo 10 production instances.

i didn't have all this snippets in my v 9 and v 10 how can i found this extra snippets

Ermin, I updated all modules multiple times in different instances and databases and the snippets just are in dev database, I still not know why.

When I run a specific database using development parameters the snippets appear.

I do not know what that should mean. From my experience (because it is easy to overlook it) I only know that it is the same, when the same modules with the same versions are installed.

Did you also check the addons folders (access rights for example) etc.?

Directory owners, permissions, users, groups?

Yes sir, is pretty estrange because development mode reads de xml files directly, and its when the snippets appear, but in productions all is into database and cache and the snippets disappear, I don't know if is a setting error or a bug, what to do? I don't know :-(

Ermin, do you have all snippets in your website builder's panel in one of your instance or database for production?

Yes, all (and even more, because I have all Odoo themes installed). But I have deployed my production server with Odoo's package installer on Ubuntu 16.04.

mmmm, I'm using git repo, may is that the problem? is some frustrating.

I don't believe so. I assume it is a deployment issue. You'll have to compare your deployments step by step...

I don't know what's wrong, I've been studying a lot of documentation before do my first deployment, and I learn how to do it better and faster, I'm almost sure that the most dificulte thing was set up the custom domain for one of my clients.

I really dont know what could be wrong in the deployment, It's pretty estrange tha my custom snippets and some snippets from odoo don't show up,a thing that I evidenced was that if I put the snnipet code directly into html editor, website interpretates the snippet, then I can configure it like if snippet have been drag and dropped from website builder's panel.

I've been lookup what provoke this issue and I found something interesting:

https://ibb.co/eZ8miF

The code html that generates odoo is this, Is some kind of bug, I really don't know what this means.

I reviewed the snippets.xml code from website module and everything looks to be ok.