SyntaxError when parsing hash with _-prefixed symbol keys in a block

[lopopolo]

This code raises a SyntaxError:

it "adds the entries from other, overwriting duplicate keys. Returns self" do
  h = { _1: 'a', _2: '3' }
end

This code does not:

h = { _1: 'a', _2: '3' }

And neither does this code:

it "adds the entries from other, overwriting duplicate keys. Returns self" do
end

Artichoke REPL (mruby 3.1.0)

$ cat crash.rb
it "adds the entries from other, overwriting duplicate keys. Returns self" do
  h = { _1: 'a', _2: '3' }
end
$ cargo run --bin airb -q
artichoke 0.1.0-pre.0 (2022-08-14 revision 6111) [x86_64-apple-darwin]
[rustc 1.63.0 (4b91a6ea7 2022-08-08) on x86_64-apple-darwin]
>>> require 'crash'
Traceback (most recent call last):
        2: from (airb):1
        1: from (airb):1:in require
SyntaxError (syntax error)
>>> h = { _1: 'a', _2: '3' }
=> {:_1=>"a", :_2=>"3"}