ALTER EXTENSION DROP FUNCTION not working ?

I'm seeing an issue with ALTER EXTENSION DROP FUNCTION
not fully unregistering the dependency of the function
on the extension. Commands to reproduce described here:
https://trac.osgeo.org/postgis/ticket/3450#comment:23

Basically I'm getting:
ERROR:  cannot drop function pgis_twkb_accum_finalfn(internal) because other objects depend on itDETAIL:  extension
postgisdepends on function pgis_twkb_accum_finalfn(internal)
 

Right after successfully running:
ALTER EXTENSION postgis DROP FUNCTION pgis_twkb_accum_finalfn(internal);

Is it a bug in PostgreSQL ? (9.3.6 running on the test machine).

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