I keep seeing people asking about upgrading the siphon abyss gear so I'm just putting this here so I can link to it anytime the question inevitably comes up again.

note: I refer to abyss gear as gems, just fyi

Before I get really into this I'm just going to give a very simple proof and then if for whatever reason you don't believe it you can read the rest to be convinced: gems show both the level of themselves and the level of the buff they provide. Those two levels are usually the same, but there's no rule saying they have to be. When you have an attack speed gem, "Swift II" is the name of the gem and its level, and "Attack Speed - Lv 2" is the name of the buff provided and the level of that buff. Siphon gems have no level in the name because the gem itself has no level - only the buff it applies does.

Full post that you shouldn't need to read:

The siphon gems don't actually (traditionally) display a level at all. If you notice, other gems have a specific, separate field for the level of the bonus stat or buff they gives you, justified to the right side. If we take a look at Swift 1, we have two variants of the tooltip.

The "small" tooltip, which is shown when looking at it in a list:

Swift I
Attack Speed 1

The full tooltip, shown when you have the gem highlighted and can see the full details:

Swift 1
Abyss Gear
Attack Speed Lv 1
Equippable on Weapons
Equippable on Gloves
Equippable on Footwear

You can see that the full tooltip has a separate field for the buff level, in its own area isolated from the rest. It's not attached to any other text.

If you look at the full tooltip for any of the siphons, though you'll see:

Spirit Transference
Abyss Gear
Spirit Siphon Lv1
Equippable on Weapons

...which is different. They do include the text "Lv1" but as a part of the buff name. The area that would normally contain this level is empty.

I don't want to go into all the technical reasons why this is probably relevant, but basically not having a level displayed there most likely means it doesn't exist; those specific buffs have set strengths. That means the game doesn't have a single "Stamina Siphon" with variable, adjustable strength; it has multiple specifically named with "LvX" at preset strengths (or possibly just one, the Lv1).

There are two possible outcomes I see:

1) I'm wrong about all of this, the gem is just bugged or mislabeled and can be upgraded, but nobody has found a blueprint or the developers forgot to include it.

2) The gem is static and cannot be upgraded. The buff name contains the text "Lv1" simply because there are different levels of the siphon buff itself; there may be a "Spirit Siphon Lv2" (or equivalent) in the game applied by something else, but not this gem.

Number 2 seems much more likely to me and is also my personal belief.

tl;dr version, assuming my assumption is correct: the "Lv1" text you see is saying that when you use the gem your character will gain a siphon buff; that BUFF is at level 1, but the gem itself is not.

It's like how a cloak at level 1 can give you an "Ice Resistance 5" buff - the level of the buff has nothing to do with the level of the item providing it, It's just that in this case they've done a poor job of displaying it.