Compute variable expressions in mustache templates: what we should get?

Given these hash and Mustache template:

Hash:

{
'a': 3
}

Template:

"This is a+2: "

Ruby and Python give me different outputs:

• In ruby, I get:

  /usr/lib/ruby/gems/3.0.0/gems/mustache-1.1.1/lib/mustache/parser.rb:286:in `error': Unclosed tag (Mustache::Parser::SyntaxError) Line 1

• In python, just empty string

Who is right? What result should we get in official mustache specification: empty string or error (or something else)? I don't know what the result is other programming languages.

from Compute variable expressions in mustache templates: what we should get?