Oh, another crappy Fox News poll. March 20-22, 2016 Fox News found Clinton up 11 over Trump. "But he wasn't the incumbent then" - I would respect that argument more if the undecided/other numbers were drastically less in this one. They aren't. 49-38 Clinton in 2016 and 49-40 Biden in this one. Says a lot people believe this one over the WaPo/ABC news one, which has little/no undecideds.
Kind of cherrypicking... from mid-summer 2015 to March 2016, the Fox poll fluctuated from Hillary +11 (the poll you mentioned) to Trump +5. Biden's lead has fluctuated between +8 and +12.