Could there be a problem when two separate parameters are using childTemplateSerializeData?
I don't see any problem with that.
Or does childTemplateSerialize data leak in some way that prohibits two consecutive renders?
Also not aware of that.
I tried it here and it seems to work as expected
https://playground.jsreport.net/w/anon/eW6NUT1e
It would be great if you can send me an updated export or guide me on how to replicate the problem.