Avoid issue in the function Series[] ?

GROUPS:
 I needed to make the first order Taylor series with the function Series. I received a second order term in the result. Mathematica version 11.0.1.0 on Mac.I think that something is wrong with the function Series[]. I take a simple function as example In[15] and want to obtain Taylor series of the first order on x,y near point (0,0). I received an expression Out[18] which has last term 0.741907xy which is the second order. A right expression Out[19] different from Out[18] can be received by direct calculating Taylor series, In[19] Clear[f, x, y] f[x_, y_] = Sin[1 + x + y]/(2 + Cos[2 - x + y]) seri1 = Series[f[x, y], {x, 0, 1}, {y, 0, 1}] // N seri2 = Expand[Normal[seri1]] seri3 = N[f[0, 0] + x*(D[f[x, y], x] /. {x -> 0, y -> 0}) + y*(D[f[x, y], y] /. {x -> 0, y -> 0})]  Attachments:
 Daniel Lichtblau 1 Vote The result is correct and is exactly what was requested in the input. If a result based on total degree is wanted, best is to a univariate substitution, as done here. In brief: f[x_, y_] = Sin[1 + x + y]/(2 + Cos[2 - x + y]); seri1 = N[Expand[Normal[ Series[f[x, y] /. Thread[{x, y} -> t*{x, y}], {t, 0, 1}]] /. t -> 1]] (* Out[276]= 0.531280931986 + 0.0361207230645 x + 0.646142403765 y *)