Hi cyaoeu,
If you keyed a pose on every frame, you shouldn’t even need UseDefaultSampleRate enabled in the first place. This sounds more like an Animation length issue. Could also be that the frame rate in your DCC is not set to 30 if UseDefaultSampleRate is becoming an issue.
I just did a test with 5 frames and 3 frames (with and without UseDefaultSampleRate) and it reimported and trimmed to 3 frames successfully.
Could you provide some more details and perhaps two sample fbx files (11 frame and 8 frame)?