[Bug Report]Table组件span-method属性合并单元格展示错误bug

[stan.lv]

Environment

windows10 view-design@4.7.0 vue@^2.5.2 node@14.15.1

Reproduction link

https://run.iviewui.com/qeyvsgwy

Steps to reproduce

表格添加三条数据，设置span-method合并单元格后，三条数据变为两条

What is expected?

正常展示合并单元格后的三条数据

What is actually happening?

应该展示三行数据的，但是合并单元格后只展示了两行