When you open the Plugin Manager available page, the plugin list takes some time to be fully loaded. However the "filter" field is enabled and the user can type there, but it's actually not applied to the list because it has not been fully loaded yet (note in the tab how the page is still loading).
The result is very confusing to the user who, in my experience, just thinks the filter is not working properly. This should be fixed by:
- Disabling the search field until the page is fully loaded.
- Applying the filter, if any, once the plugin list is completely available.