Structs do not have instaces. Rather, when trying to mock a struct, every instance is mocked. Typemock does that by using the MockAll mock mechanism. Namely, there's a single mock that is used for every use of mocking a struct of a single type.
GetInstanceMocks does not retrieve this mock by design. It retrieve instance mocks for classes. That is the reason there's no mock returned here.
Mocking a struct is possible by using MockAll<> or using the MockAllInstances property in the case of Natural mocks. It is not possible, however, to retrieve its mock following the recording. We will consider including such a feature in the future.
Please also refer to the thread on:
https://www.typemock.com/community/viewtopic.php?p=2754#2754