In sales email templates, you don’t have access to the source HTML. I just had a conversation with HubSpot support on this, and it turns out that it’s because of a line height issue that came from copy-pasting the content into the template editor.
The solution for us was to clear the formatting on the template (or just create a new template and rewrite it by hand).
Hope this helps someone else!
Chris
P.S. this was only an issue for us on some iPhones. Never android or desktop, and not all iPhones lol.