This can be fixed e.g. by adding a call to fig.canvas.draw() before computing trans_angle, but even the "old" behavior was pretty brittle -- for example, if you interactively change the figure size, the text goes out of alignment, so I'm not too worried about losing it here. The correct solution (which is robust against figure resizes) is the one implemented in matplotlib.contour.ClabelText; right now a quick and dirty solution is to just write th1.__class__ = mpl.contour.ClabelText (ugh!) but I think it would be reasonable to move that feature (e.g. as a rotation_transform=ax.transData property) to the main Text class.

Fixed by #15532.