Ambisonics group Cuts about 13dB?

Hello there.

I followed the instruction of the Ambisonics from the official PDF and found that the loudness will become about 13dB weaker than the source.

Here’s the routing in short:
Stereo Track (1) → Ambisonics (1/2/3/4OA) Group (2)–> Stereo Group (3)
By comparing (1) & (3), it’s about 13~14dB loudness loss along this chain.

Is it normal or an issue? (Nuendo 13.0.30)

Thanks in advance.