Deep variables doesn't work with variableSyntax for double curly braces

Serverless Framework
1

I’m having some problems with deep variables coming from files, but only in combination with variableSyntax with double curly braces. Example:

test2.yml:

a: HELLO
b: WORLD

serverless.yml:

provider:
    variableSyntax: "\\${{([ ~:a-zA-Z0-9._\\'\",\\-\\/\\(\\)]+?)}}"
    ...
test1:
  a: HELLO
  b: WORLD
test2: ${{file(./test2.yml)}} # test1 and test2 should be identical
custom:
  val1: ${{self:test1.a}}
  val2: ${{self:test2.a}} 

$ sls print

test1:
  a: HELLO
  b: WORLD
test2:
  a: HELLO
  b: WORLD
custom:
  val1: HELLO
  val2: '${{deep:0}.a}'

If I run the same thing without variableSyntax, and replace all double curly braces with single curly braces, I get the expected result (both val1 and val2 resolve to “HELLO”).

How can I use double curly braces and still reference child variables of a map variable sourced from a file?