Accessing child collections from record scope

I'm encountering an error when I try to reference a child collection within a table formula. I can reproduce it in pac cli, canvas apps and my own application consuming this library, so it's quite possibly my misunderstanding of the language but any advice would be appreciated.

I can successfully get the number of activities associated with a single contact using:

CountRows(First(Contacts).Activities)

However, if I try to replicate that within a collection of contacts, say all those with the first name "Mark":

Sum(Filter(Contacts, 'First Name' = "Mark"), CountRows(ThisRecord.Activities))

I get the errors:

Error 65-76: The specified column is not accessible in this context.
Error 45-77: The function 'CountRows' has some invalid arguments.

I understand that this is potentially an inefficient formula as it would require the CountRows to be executed for each individual contact, but as far as I can see the ThisRecord.Activities should be in scope at the point it's being used?

No source files or tests are named. Start by reproducing the two formulas in the reported Power Fx contexts, then trace how record scope and child-collection access are handled for the nested CountRows call; done means determining whether the formula is supported and documenting or correcting the resulting behavior.