PDF renders group incorrectly

I have a report which is rendered to PDF. When the report was run recently, the PDF showed the grouping incorrectly. When the report was run again, the grouping was correct.
Is there any way to avoid having the PDF render the groups incorrectly?

Here's an example of what I'm seeing in the PDF - in the first run, 2 items which should be displayed with id002 are displayed under id001 instead, but the total of items is shown correctly.

Run #1 - wrong

id001 name1 item 1a$1.00 item 1b$1.00 item 2b$1.00 item 2c$1.00$2.00

id002 name2 item 2a$1.00$3.00

Run #2 - right

id001 name1 item 1a$1.00


item 1b$1.00$2.00

id002 name2 item 2a$1.00

item

2b$1.00

item

2c$1.00$3.00

Is this only happening in the PDF output? What about HTML?