In the event no answer turns up with a proper research paper, I'd suggest that the evidence is mixed based on my own anecdotal experience.
I semi-regularly follow a slew of comedy hosts and can suggest that, on the comedy shows I watch at least, Sanders gets a periodic mention. It's usually a lighthearted joke, and it's about as often as other Democratic front runners except Biden. But Biden stands out for the worst possible reasons: due to his gaffs, his cringeworthy behavior with women, his harrowing record, and so forth.
I also follow a mix of wonkish shows and podcasts that range from daily news to weekly analysis of what's going on. On those, the main attractions are Warren, due to the amount of policy plans she's been receiving, and Biden, for the same terrible reason as above. (Harris was a close third that springs to mind, and it was due to her scathing record on criminal justice. She got thrashed for it so many times that I'm amazed she lasted this long into the race. Beto was up there in the list too, but mostly for the wrong reasons too. I think Kara Swisher's "he's a man-boy of the type of that made me turn into a lesbian" or something to that effect sums up the scathing commentary on him fairly well.) When a Warren policy plan gets discussed, Sanders almost always gets thrown into the conversation as a comparison point. The topic is Warren rather than Sanders, but it's a bit of a stretch to suggest that the hosts are ignoring Sanders. This may be where the rub lies, though.
As to the news outlets I'm reading, it's again Warren and Biden that stand out -- for the same reasons, and with Sanders getting mentioned for the same reasons. (One exception is the Intercept, where Sanders gets mentioned far more than any other candidate, and where it is Warren who gets contrasted instead. But the Intercept admittedly is the kind of outlet where they'd make the argument that Sanders is too right wing.)
Anyway, in short, and insofar as I'm experiencing it as a heavy news consumer, Sanders is getting coverage in my own (mostly left leaning) news bubble. But the issue is that the main time Sanders gets covered, it is because he's getting contrasted with Warren, with the latter being the actual topic. And the fix seems simple enough: Sanders would get more coverage in my own news bubble if he'd release more policy plans.
I should add that while I heavily consume news, I don't watch TV proper nor do I live in the US. So the reality on the ground, with Fox News and Sinclair group channels dominating the news landscape,
might escape me.
Update: it seems that the Intercept ran the numbers and made a video on the topic. Judging by their reporting, the main TV networks are ignoring Sanders indeed.