Why are my customer notification emails displaying as plain HTML code?

Hello, I have a problem with one of my stores. My notifications emails to my customers are being sended as code with plain HTML text. I already putted my notifications back to default. The test email and preview have no issue. Only when it is sended to an order from the store. Can someone help me with this problem? I spoke a lot of times with the Shopify support but no one could really help me.