FM delete sometimes spacers and sometimes not

DATA

1. I write<p><ph>This is a sentence. </ph><ph>This is a sentence. </ph><p>
2. I save and close the file
3. When I open it at new, I remark :

• the first <ph> still have a spacer after the point
• the second <ph> have no spacer after the point

QUESTION

Do you know where we can set in FrameMaker11 to not delete spacers ?

Other solution can be put a WHITESPACE between the elements <ph>, but it seems this causes problems while publishing.