We maintain our tags on GTM and currently tags (for different platforms i.e. Facebook, Analytics, Adwords) related to product page view are not firing for some products. I can see for these products "undefined" is returned instead of product information. All products use a single product.liquid template and here's the data layer definition:
The tag setup on GTM:
When on GTM preview mode, although using the same template, this is how it looks for two products:
I tried talking with Shopify support as it seems to be product-specific issue. Unfortunately they weren't able to provide much insights and suggested posting on this forum instead.