You can have lossless encoding with a codec compared to a bit-perfect file like WAV or PCM, and it will have reduced size as not all bits have random data.
For example, 1411 Kbps is the uncompressed stereo bitrate but you can have a FLAC file for under 1 Mbps most of the time and it's still lossless.
16 / 44.1 audio (your reference CD quality audio) is 16 bits x 44100 samples per second x 2 channels = 1,411,200 bits per second (1411 kbps) and this is still uncompressed.
Hence 1411 kbps is post uncompressing or pre compressing. FLAC typical compression ratio is 50% hence this would require bandwidth of 506 kbps ~ 0.65 mbps. This is transmission bandwidth.
Had the OP posted the uncompressed bitrate the comparison would have been perfect since he did not mention neither components of source - target. Definition wise DTS Core which is lossy is limited to 5.1 and max 1.5 mbps bitrate, so going by the OPs numbers as long as its more than 1.5 mbps and within 24 mbps (max for DTS HD MA) my guess is he is getting lossless to the extent what is in the source. I am sure he knows that but still he has doubts if there is loss, hence I requested him to post the uncompressed numbers.
Original Audio = Lossy DTS Core 5.1 (max 1.5 mbps) + additional data (to make up for the loss while encoding to DTS 5.1) and channel data (additional 2 channels beyond 5.1 to make it 7.1)
Bitrate remaining same but no surround sound even though it's showing multi-channel
Hope your did not select LPCM audio from the Blue Ray Disc menu and tweaked only BD Player menu. This means the bitrate being displayed by your AVR is post uncompressing, so I would assume its the lossless you are getting. Not sure why the 2 channels are not coming, see if center channel is coming if not LPCM is stereo and multichannel, more investigation is needed. I may suggest go to
https://www.blu-ray.com/ and locate your specific region and disc variant and see what is the audio bitrate shown, we can extrapolate from there what should have been displayed.