Although Matlab is closed source, many functions rely on well known code. Examples:
When such bugs appear in the published releases, the testing team has obviously overseen them. The corresponding unit-tests must be missing or incomplete. This is a great pity, but cannot be avoided, because bugs are an inherent feature of large complex systems.
The Matlab users will find such bugs and report it either directly to the technical support, or they post it here in the forum.
The list of published bugs (see e.g. the bugs FAQ) follow this policy:
Published bug reports are for bugs that have been reported in this system that may be of interest to customers, based on general use of our products.
As far as I understand this, there are also known bugs, which are not published, because TMW thinks, that they are not in the interest of the customers. Unfortunately the customers do not agree from time to time and it is hard to define, what the "general use of [the] products" exactly cover. In all of these cases (5 times for me personally) the TMW team has been very active in clearing the problem and finding workarounds. In addition this forum and CSSM help frequently users to find out, why an assumed bug is an expected feature: While bugs are the nature of software, misunderstandings are the nature of human users.