Channels are added individually to the bridge, so the real reason is further down:
[Jul 21 12:14:08] DEBUG[11955][C-00000009]: bridge_native_rtp.c:334 native_rtp_bridge_compatible_check: Bridge ‘b25a204f-aea7-4cd1-abed-d7b38693970d’ can not use native RTP bridge as channel ‘SIP/1402-00000013’ has features which prevent it
Which will occur if there is something on one of the channels which needs to get access to the audio stream. WIth chan_sip I’m not sure what that would be.