Hi
The Pixel Manager doesn’t change any elements of a theme.
It only injects. <script> tags for products. Those <script> tags contain objects with product information. Those <script> tags don’t include any executable code that could change the HTML of the website in any way.
However, there are themes that don’t ignore <script> tags in their custom render engines, as they should (and as per HTML specification).
E.g. this was a known issue in Elementor that was fixed by Elementor. Here’s more information about it: https://sweetcode.com/docs/wpm/troubleshooting#elementor-related-products-and-upsell-products-widgets
This is a theme issue and needs to be fixed by the theme developer.
If you want to verify this, switch to a standard theme like Storefront to confirm.