I'm having the same issue. GPS connector set as mock location app, connected to satellites and position correct. tried with location rerouted to OS and also with play services location, every iteration of setting possible, no location available in any maps app I have tried. only difference in my setup is i'm using a USB GPS dongle connected to a USB hub.
I assume that there is something in the code of the stock android release for Pixel tablet that is different from the phones, due to the fact that the tablet does not have built in GSM, so it has a version of android that is Wifi only, and lacks some dependencies required to allow for GSM mock location. Another sign this is the case is that when trying to install Waze from aurora store an error message appears saying that this app is not available for your device (lacks GSM antennae)
I believe GOS team has made clear that they plan to introduce a Location Scopes feature to allow for Mock Location on a per app basis, which is extremely useful sounding. Whether or not they plan to address the lack of capability inherent to the version of android on the pixel tablet in particular is another story. Would love for a member of the GOS team to weigh in on this and give an idea of when this functionality would be introduced.