Dropping, selling or backpacking Phase Boots immediately removes the „Phase buff“ that Phase Boots’ active ability provides.

Other items do not behave like this and either still provide their temporary buffs when the respective item gets sold, dropped or backpacked (e.g. shadow blade) or provide a downgraded version of their temporary buff (e.g. silver edge doesn’t break if dropped, sold or backpacked).

Additionally, when owning more than one pair of Phase Boots, selling, dropping or backpacking ANY Phase Boots will remove the buff as well, even if the pair of Phase Boots you used is still in your inventory.

Helm of the Dominator suffers from a similar issue (aura is still provided, but the dominated creep dies if ANY owned HotD is sold/destroyed).

Phase Boots should either provide the full buff (like shadow blade) or at least provide the movement speed bonus when dropped, sold or backpacked (just like Silver Edge still provides invisibility, movement speed and the physical proc, but doesn’t break when outside of the main inventory).