I have the same issue with my Essential PH-1 and Android P.
If I try to directly install from openandromaps, Oruxmaps starts and either I get the message that map download failed, or I see the download notification but there is no progress and nothing is downloaded.
It seems to me that the issue came with one of the latest Android P patches, though, because I regularly update my maps and I did not have any issues in August with Android P.
Oruxmaps is clearly outstanding and the most awesome app I have on my phone!
Eriol