I am adding custom attributes to the Unreal Crash Reporter with the __sentry game data key, but I am not seeing that information show up.

When custom attributes are added, they appear in the CrashContext.xml file that gets uploaded to Sentry along with the Crash Report. If there is data missing in Sentry, it's likely that the data is missing from the CrashContext.xml file that is outputted by the Crash Reporter as well.

One situation where this can occur is when an internal string buffer is filled in the Crash Reporter and ends up truncating everything else.

Try increasing the CR_MAX_DYNAMIC_BUFFER_CHARS in GenericPlatformCrashContext.h to a higher value to see if data starts coming through.

Note: It is possible that there are other causes of this particular issue regarding missing attributes.