Bug introduced in 10.0.2 or earlier and fixed in 11.1.0
Note: TransformedRegion was introduced in 10.0.0.

Reproducing an example from Mathematica's help, I obtain an incomplete plot

ClearAll; a = TransformedRegion[Disk[{1, 1}, 4], {Indexed[#, 1] *Indexed[#, 2], 
Indexed[#, 1] + Indexed[#, 2]} &];RegionPlot[a]

instead of an entire plot. How to fix it?