Ambiguous syntax when using scalar subquery without parentheses in JSON_OBJECT

This works as expected:

select json_object(
  key 'a' value (select 1), 
  key 'b' value (select 2)
)

And produces:

{"a":1,"b":2}

Curiously, this works as well:

select json_object(
  key 'a' value (select 1), 
  key 'b' value select 2
)

It seems the parentheses around the scalar subquery of the last key/value of a JSON_OBJECT are optional. This, however, fails due to the ambiguity of the , operator:

select json_object(
  key 'a' value select 1, 
  key 'b' value select 2
)

The error being:

SQL Error [42001] [42001]: Syntax error in SQL statement "select json_object(\000d\000a  key 'a' value select 1, \000d\000a  [*]key 'b' value select 2\000d\000a)"; expected "*, INTERSECTS, NOT, EXISTS, UNIQUE, INTERSECTS"; SQL statement:
select json_object(
  key 'a' value select 1, 
  key 'b' value select 2
) [42001-210]

There seems to be a bug either in the second or in the third query. I tend to think that the second query is wrong, and the scalar subquery without parentheses should be rejected, but then again, this works:

select select select 1 = select 1

So it doesn't seem to be a simple fix.

Reproduce the three JSON_OBJECT queries and the nested SELECT example in H2 to compare which forms the SQL parser accepts. Review the parser entry points for JSON_OBJECT key/value expressions and define whether unparenthesized scalar subqueries should be accepted or rejected; done requires consistent handling of the ambiguous comma syntax and regression coverage for these examples.