The 2019 MacPro when paired with an Avid HD Native or HDX card has a known problem with enumeration. The more channels of I/O that are present, the more likely it is to occur. The primary symptom is that one of the instances of an HD I/O will disappear on boot. In this state the ProTools audio engine will not operate correctly, and there may be clocking irregularities (i.e. not all connected interfaces switching to the same sample rate).
To restore normal operation, you can close ProTools, power cycle all connected interfaces and then launch ProTools again.
This problem happens when one or more HD Native or HDX cards are installed directly into the PCIe slots of the MacPro. With a single card installation, there may be better results by putting the card into PCIe slot 2 of the computer. Otherwise, the symptoms go away entirely if the Avid card(s) are placed into a thunderbolt expansion chassis, instead of installed directly.
If one of thee solutions is not possible, we recommend launching Audio Midi Setup after boot. If the HDX or HDN is showing the number of channels suspected, it is OK to boot ProTools. If it is NOT showing the correct number of channels, power cycle the interfaces until it is showing the correct number, then launch Protools.