The issue is that when you create a new album of disparate tracks pulled from different albums, the individual tracks are very likely to be at different volume levels.
Album-leveling mode is designed to preserve the differences in level between each track on an album.
So if you have a track with an average level of -10 LUFS, and a track with an average level of -30 LUFS, there will be a 20 dB difference between them if you add them to a custom "album".
If you added them to a playlist, rather than creating an album from those tracks, track-based leveling would be used, so the first track would be attenuated by -13dB and the second would be boosted in volume by +7dB so that both at the leveling target of -23 LUFS.
You cannot use the existing "compilation" tag, because many compilations crossfade tracks into one another so they must all be played at the same level.