BeforeInitialGet seems to only run once on combined data stores, but only sometimes

No known repro case, it's happening in my game where:

```
local store = DataStore2("someCombinedKey", player)
store:BeforeInitialGet(callback)
print(store:Get()) -- The result of BeforeInitialGet
print(store:Get()) -- The original data?
```

`BeforeInitialGet` is only ran once, as observed by a print inside.

Start with the combined DataStore2 path and the BeforeInitialGet and Get entry points shown in the report. First reproduce the two-call snippet in a minimal Roblox setup, then determine whether the callback is expected to run once or whether the second Get returns incorrect data; done means a confirmed cause and a reproducible regression case.

Written by the indexing model from the issue text.