Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 745/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 398.11
Broken: version: 2.90 (sub 3), branch: master (modified), commit date: 2020-05-26 15:15, hash: rB1f223b9a1fce
Short description of error
Joining areas with some complex arrangements can result in invalid edges
Exact steps for others to reproduce the error
In the following blend you will get a bad result if you join the editor at the bottom-left to the one directly above it.
Similarly, the following video (provided by @Carlos Muñoz (carlosmu) ) illustrates similar behavior:
Note that I do have a perfect fix for this, but I wanted to have a bug report on file so this issue doesn't get lost (again).