Hmm, that sounds like I'm wrong then(as it respects that option too), apologies if that's the case. Console-UI was BTW fixed in v2.0.4 for skip priority actually being skip instead of low. I look into it again latter today/evening and post back. First I couldn't reproduce(yesterday), and then tried your settings and then could, so suspected it was about the full allocation option, but disabling that, still could reproduce, so narrowed it down to that other option before mentioned, in two extra on/off testings each, but your report just sounds like possibly unrelated. BTW, that 2.1.2-dev0 I'm pretty sure just is 2.1.1, like mine, and just an issue happening at times during the building process, with naming screwup. Anyway, will find out in-depth, later.
Edit: You're right, in console-UI that option isn't hindering functionality, whereas it does in GTK-UI and web-UI. I'll investigate this evening, or tomorrow and find out why exactly.
Edit2: The reason for former, I found was something I also actually had
reported years ago, but apparently forgot. The rewritten deluge-console in deluge2 doesn't apply/save settings in the curses preferences pane to session correctly(i'll see if can find culprit for this, and add to report, later), so when I tested before, I had the prioritize setting disabled, and it worked fine, and then when enabled it and retried, then again worked fine, though as said, I forgot that the option effectively wasn't even enabled in libtorrent(reentering preferences pane looks like it's set, but if restarting console-UI and then rechecking, then will see it's really not). However that would indicate that when you tested it previously yourself in deluge-console and found working, then would have had the option disabled too, before starting deluge-console, or disabling it through cli/non-interactive mode, and not through the curses pane(it's easy to see if option enabled or not, from checking for existence of hidden dot "parts" file, next to download payload, as can only check core.conf after deamon stopped usually, where settings saved from memory to disc). Anyway, I highly suspect then, that the different libtorrent versions show different behaviour regarding this option, or more specifically libtorrent v1.2.x vs v2.x branches presumably. Hope helps.
Edit3: Last, indeed no issue anymore with that option when changing to libtorrent 1.2.x, where I tested latest v1.2.19 on deluge 2.1.1, without issue. Btw, it's seemingly deluge itself which in 'core.torrent.py' defines upped piece-priorities for first/last pieces and applies said generated defenition list through a generic prioritize_pieces libtorrent option, so either this is an issue with said generic prioritize_pieces libtorrent 2.x option, or instead a deluge issue of generating the arg correctly(list of changed piece priorities for torrent) to that option for libtorrent 2.x.