This is one of WP’s most annoying “features”. The text editor used to be called the html editor, which was infuriatingly misleading, because it did/does not show, nor allow, all of the html. ( so what’s the point- if I want to see just the text, it’s in the visual editor already.) The solution to that seems to have been to rename it ‘text editor’ -problem solved. I understand that stuff could be added that will cause problems- but it’s YOUR website. If you want to tinker under the hood, and it blows up, so be it.
There is a way to stop this code stripping, without using one of the plugins for that, but it involves making changes to the functions.php file, which sounds like it will probably also be off limits to you in your particular environment. see this: https://www.leighton.com/blog/stop-tinymce-in-wordpress-3-x-messing-up-your-html-code/
My suggestion is to find a way to insert your google map in a widget area, where the iframe code can be entered in a text widget, and it will work.