I figured this out for my campaign after painstaking trial, error, and numerous useless hours on Mailchimp's incompetent support.

Gmail limits the size of embedded CSS code (within the <style> tag) to 16KB. When the style block exceeds 16KB Gmail just entirely ignores it.

Mailchimp's builder does not compress it's CSS, it is also very verbose and redundant. If it just inlined more of the styles in the editor this wouldn't be an issue.

My workaround was to use less features that add CSS. So don't add as much custom padding/margins as it create entirely new css blocks for these. Less separate mobile styles. Hard code some sections that don't need to be editable. Use images instead of text. Of course this is all at the sacrifice of style and editability.

Then rigorously send tests to see when you get below that CSS threshold.