This won't be fixed due to the following message from our developers:
This is an unsupported usage of SystemAPI. As SystemAPI only works if contained directly in a struct implementing ISystem (or systembase). This limitation extends even to partials. So just because one struct implements it, doesn't mean the other struct knows about it. This all said, ISystem can be implemented by both partials, which should work.
Dont do:
partial struct TestSystem : ISystem {} // file-a.cs
partial struct TestSystem { /* SystemAPI usage inside */ } // file-b.cs
Do:
partial struct TestSystem : ISystem {} // file-a.cs
partial struct TestSystem : ISystem { /* SystemAPI usage inside */ } // file-b.cs