Using merge field on OLX email causing responsive layout to fail in mobile view

I'm finding when using a merge field when building an email on Online Express, it's causing the layout of the email to show up differently on a mobile. Without a merge field, the layout is responsive and text is a readable size on a mobile phone. When I sue a merge field, eg “Dear (first name)”, it causes the layout of the test email in my mobile inbox to look like a desktop email, making everything too small. I've tested this with other people and it still happens. Can anyone help?

Thanks