Whatever the screen of the DS DAC shows you at any point in time, that’s the digital audio signal format that it is receiving in that moment. If it says DOP DSD64 then that’s exactly what you have on the wire. Your original question is about why does it sometimes say PCM 44.1/24, and what I’m trying to tell you is that when it says it’s getting PCM 44.1/24 then it really truly is getting that.
The DS DAC I2S inputs only make use of two signal lines instead of the usual three: the serial data line and the word clock line aka left-right clock. The word clock switches high or low to indicate the start of each new sample appearing on the data line, alternating between a sample for the right channel and one for the left channel. In the intervening time some number of bits will be signalled on the the data line – most often 16 or 24 or them, but other sample depths are possible.
When the incoming audio is sampled at 44.1kHz the word clock will toggle high and low 44,100 times per second. The DAC can directly observe this, and won’t be confused about it. If it says 44.1 on the screen, the signal being received is sampled at 44.1.
DOP is a clever scheme to pack 16 bits of DSD data inside of each “sample” in a 24-bit PCM transmission system in a way that makes it obvious to a DOP-compliant receiver that the real audio data is DSD not PCM. To match the data transfer rate of DSD64, it is necessary to pack the bits inside a 24/176.4kHz PCM system. When coming over I2S this means the word clock will be switching 176,400 times per second. Again there is no opportunity for confusion at the DAC end. If it sees this sample rate, and recognises the pattern embedded in the first 8 bits of each “sample” as the one which signals DOP encoding, it will process the audio as DOP and let you know on screen.
So what I’m trying to tell you is that when the DAC says it’s getting 24/44.1 PCM that’s what the computer is sending it. And if you don’t want it to be doing that, you’ll need to talk with people who understand how Audirvana interacts with macOS to try and figure out how to ensure you get the DOP transmission from the computer that you’re looking for.