Thanks for this Andre.
It said "sure, asking Poweramp to play [album] by [artist]", has a touch prompt with 2 options, one with the name of the album, and one asking to open Poweramp (kinda). I did nothing (much of the point of this is to be able to do it hands free) and it opened '98 Freestyle by Big L, which might be the first track by track name ('98) though I'm not sure. And it didn't play it, just opened Poweramp (which had been open but paused in the background).
Tried again, clicked first option, it opened [trackname] which is the same as [album], but by a different artist (asked for Olympia by Gorgon City, got Olympia (track) by Sundara Karma. Also didn't play).
Tried the second option, worked the same as the first.
Tried "play Olympia album by Gorgon City in Poweramp", same result as before. If I click the second option "open Poweramp", it opens Olympia by Sundara Karma and plays it. Maybe because it was already open though.
Let's try something unambiguous.
"sure, opening Harvest by Neil Young in Poweramp.... .... .... something went wrong. Please try opening the app to continue"
Closed google assistant & Poweramp. Trying again.
"ok, asking Poweramp to play Harvest by Neil Young"
I have to click the top option, goes to Harvest (song) from Harvest (album) by Neil Young. I have to click play.
Let's try again.
Turns out the first option isn't an option it's a title bar. One has to click the "open Poweramp" each time.
Trying OK Computer by Radiohead. 3rd try, opened Poweramp. Harvest by Neil Young continues.
Looks like it doesn't like albums, maybe tracks only.
Paranoid Android by Radiohead: played the second version of the song (heads on the radio, live & bootlegs) rather than the first version in the list, from OK computer. Illogical. List seems to be tracks ordered by name, all from the same artist.
"play the album OK computer in Poweramp": something went wrong. (repeated, does the same every time, even though it knows it's al album by Radiohead even if I don't say album nor Radiohead. If I then click on 'open the app', it continues playing the previous song (paranoid android bootleg).
Ok, giving up. I presume this is google assistant's fault?