(the problematic point is org.eclipse.xtext.parser.antlr.AbstractIndentationTokenSource.computeIndentation(String)

	protected int computeIndentation(String text) {
		int result = 0;
		for(int i = text.length() - 1; i>=0; i--) {
			char c = text.charAt(i);
			if (c == '\n' || c == '\r') {
				return result;
			}
			if (c == '\t') {
				result+=getTabWidth();
			} else {
				result++;
			}
		}
		return -1;
	}

I am note sure if it a bug. Please file a ticket. Anyway.. Why dont you simply throw away and use a ws rule that allows newline space and tab

You can increase decrease indention only after a newline. This is
Why xtext looks for these newline. Can you give it a try with a simplyfied language

https://bugs.eclipse.org/bugs/enter_bug.cgi?product=TMF&component=Xtext

the following works for me ?!?

grammar org.xtext.example.mydsl3.MyDsl hidden (WS, SL_COMMENT)

generate myDsl "http://www.xtext.org/example/mydsl3/MyDsl"
import "http://www.eclipse.org/emf/2002/Ecore" as ecore


file_input:
	{file_input}
 ( los+=stmt )*
 ;
 
 if_stmt:
 'if' tests+=test ':' suite+=suite
;

test:
	value="true" | value="false"
;
 

suite:
 simple_stmt=suiteblock
;

suiteblock:
	BEGIN 
		stmts+=stmt+ 
	END
;

stmt:
	print_stmt | if_stmt
;

print_stmt:
	PRINT vlaue=STRING
;

 
terminal BEGIN: 'synthetic:BEGIN';
terminal END: 'synthetic:END';
terminal PRINT: 'print';

terminal STRING: 
			'"' ( '\\' . /* ('b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\') */ | !('\\'|'"') )* '"'? |
			"'" ( '\\' . /* ('b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\') */ | !('\\'|"'") )* "'"? |
			'"""' -> '"""'?;


terminal SL_COMMENT: '#' !('\n'|'\r')* ('\r'? '\n')?;

terminal WS: ('\r'|'\n'|'\t'|' ')+;

if true:
    print "x"
    print "x"
print "y"

@testdec
def x():
    pass