Fail to Create Render Target 2D with Autogenerate Mipmaps on

Hi there, I hope you can help me with this one (midgunner66, Arkiras and Bits360 me helped a lot getting to this point):

I am creating a render target from a downloaded texture. However, whenever the Render Target 2D size is over 256 x 256 px and Autogenerate Mipmaps is on, the final result is an empty/black texture. It does work for outputs 256 px or smaller (or any size, if Autogenerate Mipmaps is off).
Thank you!

EDIT: it seems that even in 256 x 256 px mipmaps are not being generated:

338430-005f788a1e1e99491640ae9893aca08a824b0a7b.png