Hi there,
Sasha here from Metafields Guru team.
That’s correct, multi line text metafields do allow merchants to add HTML as metafield values. The issue is a bit different. There is an issue with HTML formatting for metafields used as “dynamic data source” in Online store 2.0 themes.
I’ve created a multi line text metafield with a HTML table for value on my test store:
Next, I created a definition for this metafield:
And it got HTML code displayed as plain text too:
So I edited theme code manually and replaced the code generated by Shopify with proper Liquid code generated by our app ( https://apps.shopify.com/metafields-editor-2 ) : 
{{ product.metafields.custom_data.nutrition_information.value }}
and it worked like a charm:
Apparently, Shopify generates code that fails to render HTML correctly. Luckily, you can avoid running into this issue with a pretty simple workaround: you can use Liquid code reference of a metafield instead of connecting it as a dynamic data source. Or, if you’re familiar with theme customization, you can place the Liquid code directly in a product page template file (or any other template page: collection, blog post, CMS/static page etc).
We’ve got a great step-by-step tutorial showing how to display a metafield on product page for both Online store 2.0 and non-OS 2.0 themes on our help portal: https://support.metafields.guru/support/solutions/articles/44002276080
Hope this helps ! 
Best regards,
Sasha Kachkovskyi
Co-founder and CEO
256 Development