Servlet filters not invoked for _am/api/discovery/* URLs? - web.xml

Question

I'm updating a GAE application to the Java8 Cloud SDK environment; I'm also updating it to use Cloud Endpoints version 2.
My app registers some servlet filters in its web.xml file, one for Objectify and one to do some initialisations such as creating singleton instances of some utility classes the app uses.
This is an excerpt of web.xml:
<servlet>
<servlet-name>EndpointsServlet</servlet-name>
<servlet-class>com.google.api.server.spi.EndpointsServlet</servlet-class>
<init-param>
<param-name>services</param-name>
<param-value>com.myapp.service.Service</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>EndpointsServlet</servlet-name>
<url-pattern>/_ah/api/*</url-pattern>
</servlet-mapping>
<filter>
<filter-name>InitializerFilter</filter-name>
<filter-class>com.myapp.InitializerFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>InitializerFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>ObjectifyFilter</filter-name>
<filter-class>com.googlecode.objectify.ObjectifyFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>ObjectifyFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<!-- Add a filter that fetches the service config from service management. -->
<filter>
<filter-name>endpoints-api-configuration</filter-name>
<filter-class>com.google.api.control.ServiceManagementConfigFilter</filter-class>
</filter>
<!-- Add a filter that performs Endpoints logging and monitoring. -->
<filter>
<filter-name>endpoints-api-controller</filter-name>
<filter-class>com.google.api.control.extensions.appengine.GoogleAppEngineControlFilter</filter-class>
<init-param>
<param-name>endpoints.projectId</param-name>
<param-value>${appId}</param-value>
</init-param>
<init-param>
<param-name>endpoints.serviceName</param-name>
<param-value>${service}-dot-${appId}.appspot.com</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>endpoints-api-configuration</filter-name>
<servlet-name>EndpointsServlet</servlet-name>
</filter-mapping>
<filter-mapping>
<filter-name>endpoints-api-controller</filter-name>
<servlet-name>EndpointsServlet</servlet-name>
</filter-mapping>
It looks like filters correctly kick for all URLs (for example, URLs handled by some other servlet that I instantiate but are not shown here), but not the _ah/api/discovery/* URLs that implement Google's nifty APIs Explorer tool.
No exceptions are thrown at deployment.
Note that I already tried changing the <url-pattern>/*</url-pattern> to <url-pattern>/_ah/api/*</url-pattern>, to <url-pattern>/_ah/api/discovery/*</url-pattern> and mapping using <servlet-name>EndpointsServlet</servlet-name> rather than a URL pattern, to no avail.
Awkwardly enough, the very same configuration did work on Friday morning, then after a redeployment made later in the afternoon it stopped. And I'm pretty positive I did not change anything.
Is this known behaviour? For example, this may be because the API Explorer is "stitched on" the endpoints URL externally rather than being part of EndpointServlet itself?
Otherwise, what am I doing wrong?
=========
I fixed this problem by moving the logic that was in the filter to a ServletContextListener, and this made the app stable. This is only viable for once-for-servlet-lifetime initialisations, of course, so the question still stands: is the cloud API Explorer expected to trigger the servlet filters registered on EndpointsServlet?