### Version 2.7.14 ### Reproduction link [codesandbox.io](https://codesandbox.io/s/style-merge-9hltuv) ### Steps to reproduce click mStyle button twice ### What is expected? show red border-right and blue border-bottom ### What is actually happening? only show red border-right. is not similar as setAttribute but vue3 is similar like setAttribute --- is this a bug? <!-- generated by vue-issues. DO NOT REMOVE -->