[formatting2] Serialization-based formatting leads to insertion of tokens

I'm experiencing a strange issue where formatting my DSL leads to insertion of parentheses. I suspect there is something going wrong with serialization.

Minimal repro: https://github.com/REGnosys/rosetta-dsl/tree/formatting-bug2

The language in the repro consists of an "expression" language with only a single operation (<Expression> exists) and parentheses. The operand of the exists operation is optional.

Problem: formatting the following code with the default(!) formatter

exists exists

results in

( exists ) exists

This does not happen if useSerializer of the FormatterTestRequest is set to false.

Grammar:

grammar com.regnosys.rosetta.Rosetta with org.eclipse.xtext.common.Terminals
import "http://www.rosetta-model.com/RosettaExpression"

RosettaExistsExpression:
	RosettaCalcPrimary
	(
		=>(
			({RosettaExistsExpression.argument=current} 'exists')
		)
	)*
	| // Without left parameter:
	(
		(
			 ({RosettaExistsExpression} 'exists')
		)
	)
	(
		=>(
			({RosettaExistsExpression.argument=current} 'exists')
		)
	)*
;

RosettaCalcPrimary returns RosettaExistsExpression:
	'(' RosettaExistsExpression ')'
;

Formatter:

class RosettaFormatter extends AbstractFormatter2 {
	override format(Object obj, IFormattableDocument document) {
		
	}
}

Failing unit test:

@Inject
extension FormatterTestHelper

@Test
def void parenthesesBug() {
        assertFormatted[
	        it.useNodeModel = false // test succeeds if replaced with `it.useSerializer = false`
	        it.expectation = '''
	        exists exists
	        '''
	        it.toBeFormatted = '''
	        exists exists
	        '''
        ]
}

Start with the FormatterTestHelper test and the FormatterTestRequest setting that switches between useNodeModel and useSerializer. Compare the two formatting paths using the supplied grammar and minimal reproduction. Done means the default formatter preserves exists exists without inserting parentheses.