Fonts are parsed in one way but rendered in a different way

Nginx 1.10.3, WordPress 4.9.1, Elementor 1.8.11. A subdomain site.

The problem

Global fonts appear fine inside Elementor view but not outside of it. That is to say: When you click "Edit this page with Elementor" global fonts like Assistant for headings or Open Sans Hebrew for general text appear fine but the moment you quit, all the site is in Arial.

Here's what I did trying to solve it:

1. Unselecting the option that disables Elementor global fonts, and saving the changes.
2. Clicking "Regenarate CSS", and saving changes.
3. Flush all caches in site (cd /var/www/html/test.example.com sudo wp cache flush).
4. Flush browser cache.

I've checked the course with both TwentySeventeen and my own theme (Blankslate). The problem doesn't seem to be theme related.

Notes

• This problem happens from three browsers (Firefox, Chrome, Edge).
• This problem is extremely wired given that in HTML source code, the fonts are parsed correctly (inspection shows Assistant for headings or Open Sans Hebrew for general text) and yet it is rendered differently, in Arial.
• The site's theme is Blankslate - the most minimal theme ever and there are no CSS customizations whatsoever.

My question

Let me clarify that this is not a question about the pagebuilder Elementor. This is a question about the principle of why fonts are parsed correctly but rendered otherwise.

What might cause that?