I just found another bug in the flexible query parser (together with Robert Muir, yay!).
The following query string works in the standard query parser:
(field:[1 TO *] AND field:[* TO 2]) AND field2:z
+(+field:[1 TO *] +field:[* TO 2]) +field2:z
The flexible query parser though yields:
+(field:[1 TO *] field:[* TO 2]) +field2:z
Test patch is attached (from Robert actually).
I don't know if it affects earlier versions than 3.5.