When plotting the mesh there are some lines missing. In function plot.mesh for every element a line is plottet between (p1) to (p2), (p2) to (p3), ... (pn-1) to (pn). There should be an addiational line between the last an the first vertice (pn) - (p1) to plot the whole element.
for el in isa.elements:
pts = [list(i) for i in zip(*[v.xy for v in el.vertices])]
ax.add_line(Line2D(pts[0], pts[1], color='b', ls='-', lw=0.25))
ronaldtanner
commented
2 years ago 
When plotting the mesh there are some lines missing. In function plot.mesh for every element a line is plottet between (p1) to (p2), (p2) to (p3), ... (pn-1) to (pn). There should be an addiational line between the last an the first vertice (pn) - (p1) to plot the whole element.