IMO, everything may be working correctly, but the powerline adapters could be a bottleneck.
If you want to troubleshoot, move the pi and plug it directly into the router to see if performance improves. This will indicate if the powerline adapters are the bottleneck. You can also test the adapters by copying a large file from the usb drive to the Brix machine over the network. Ideally you should see speeds almost equivalent to having the usb drive plugged into the Brix, which also could be done as a test or alternate permanent option.
FLAC's use a lot of bandwidth, especially hi-res. If you can set up wireless to get a good signal upstairs that may be a better option than the powerline adapters. If all network/internet is riding on them they need to be very good ones to handle the load gracefully. They will vary in performance depending on many factors.
If you can't overcome the bandwidth limitations, set the MC server for high bandwidth MP3 and give it a shot. That's probably how the library was being served when it worked "better".
EDIT: I saw in another thread that 24.0.15 has DLNA issues. Make sure you are running .19.