Rules compilation failure depending on condition ordering

When creating rules in guided editor, we are running into validation errors depending on the order of the conditions. The problem is that the order of the conditions is important:

when
  COND_X
  COND_Y   => works

when
  COND_Y
  COND_X   => fails

java.lang.AssertionError: [10,18]: [ERR 101] Line 10:18 no viable alternative at input 'Number' in rule "ATO_Negative_AU_Test"
[0,0]: Parser returned a null Package

Test cases showing the compilation failures (@Test testOrder1 vs
testOrder2) can be found here:
https://gist.github.com/martinweiler/e684a0e75a2c9f4de4dee5a13affb8b0