jQuery Grid Plugin - jqGrid

I've run into a bit of an odd case, and I wonder if someone might be able to help.

In the code I am working with, I have been using multiple level (2) grouping for several months now, and without any problems.  I recently had a request to add a third level of grouping, which caused the furling (+/- toggling) on that level to behave oddly.

The three levels, by groupField name are, respectively: part_description, locn_abbr, and corporation_abbr.  corporation_abbr is the recent third level addition.  Example output looks like this:

1.1 Part Description 1

1.2 Location 1

1.3 Customer 1

2.1 Part Description 2

2.2 Location 2

2.3 Customer 2

3.1 Part Description 3

3.2 Location 3

3.3 Customer 3

Clicking + next to Part Description or Location will, as expected, expand the data below Customer.  Clicking - next to either also works properly, with the data rolling back up to the appropriate level.

Clicking + next to Customer expands the data as expected.  Clicking - rolls up the current entry's customer data as well as the next entry's customer data, which also removes the next entry's Part Description and Location levels.  Using the above example, assuming I've clicked - next to the first entry's customer level, it would look like this:

1.1 Part Description 1

1.2 Location 1

1.3 Customer 1

1.3 Customer 2

3.1 Part Description 3

3.2 Location 3

3.3 Customer 3

Additionally, if I click + next to 1.3 Customer 2, that entire entry is redrawn as it was before, below entry 1.

A snippet of the grouping code is located at http://codetidy.com/4358/

I've googled this problem with every conceivable search term, but haven't found anything.  When that didn't work, I looked at the source, and messed around with groupingToggle a bit.  I didn't have any success with that, either.  Does anyone have any insight into this problem?  Is there something obvious I'm missing in the code snippet I provided?

Thanks in advance.