Posting my solution as an answer so that someone having this problem in the future doesn't have to dig through the comments above.
As @DamyanOgnyanov pointed out in the comments to my question, the necessary rules to infer types based on rdfs:domain and rdfs:range are not included in GraphDB's RDFS-Plus and RDFS-Plus (Optimized) ruleset.
They are, however, included in the RDFS ruleset, which is counter-intuitive because RDFS should be the basis for RDFS-Plus.
In order to make the RDFS-Plus ruleset a proper extension of the RDFS ruleset and get support for rdfs:domain and rdfs:range , I added the following rules of RDFS to RDFS-Plus. The ruleset file can be found at <your-graphdb-folder>/configs/rules
Id: rdfs2
a b c [Constraint b != <rdf:type>]
b <rdfs:domain> d
------------------------------------
a <rdf:type> d
Id: rdfs3
a b c
b <rdfs:range> d
------------------------------------
c <rdf:type> d
Furthermore, I also added the rules with IDs rdfs6, rdfs7, rdfs12, rdfs13 from RDFS to RDFS-Plus.
I did not add rules rdfs5, rdfs9 and rdfs11. Rules rdfs5 and rdfs11 are covered by the transitive property rules and rdfs9 is covered by the axiom and rules about psys:transitiveOver.
Edit: GraphDB did not pick up these changes when I created a new repository with the edited RDFS-Plus ruleset. I had to select "Upload custom ruleset" and upload the ruleset that I had edited (i.e. the RDFS-Plus ruleset which still has the default name).