v-once on template not working inside a v-for in two cases

[posva]

Version

2.6.10

Reproduction link

https://codepen.io/xiangyuecn/pen/eYmmPNP

moved from #10892

Steps to reproduce

The repro adds entries to an array, causing a re render that. The template intentionally displays the elapsed time to see which v-once work and which do not

What is expected?

All v-once to never render again

What is actually happening?

Test 3 first template with a v-once re renders all the time. Workaround is to append an empty span: <template v-once>[{{ getTime() }}] {{ obj.msg }}<span/></template>
Test 5: the second template with v-once still rerenders. Workaround is to use a different tag like a span or use the v-once on the parent.