Sub categories in OpenCart - "disappear" sometimes? bug?

I have OpenCart installed for a clients e-commerce shop.

The shop has several categories and a few sub-categories. When they have been set up its fine but without any obvious reason, sometimes when you go to the Opencart admin and view the categories, the sub-categories cant be seen (or the main Parent category cant be seen).

E.g. and it will appear something like this

Bikes
Electric
Hybrid
Road
Children


To "fix" it, all you have to do is click Edit on any of the categories, make NO changes, click Save and then its all fine and you see the categories and sub-categories.

E.g.

Bikes
Bikes > Electric
Bikes > Hybrid
Bikes > Road
Bikes > Road > Childrens
Bikes > Road > Adults

I have not updated the installation since the shop went live only a few weeks ago and I know its been updated a lot recently so its still on v1.4.9.3

It's probably this bug here:

http://forum.opencart.com/viewtopic.php ... 94#p124494

Unfortunately, that doesnt fix the problem.

Made the changes and its still doing the same thing