dynamic v-slot maybe overwite when used together with v-for on same element

[zrh122]

Version

2.6.10

Reproduction link

https://jsfiddle.net/zrh122/scg6eq1t/

Steps to reproduce

1. open the reproduction link
2. look at the rendered result

What is expected?

a: a
b: b

What is actually happening?

a:
b: b

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This is an english version of issue #10165.