To quote the relevant parts:

WEAPON JAMS
Weapons may jam either through extreme age, maltreatment of
their machine spirit, or just plain poor design. An unmodified
result of 96 to 00 on a Ranged attack, in addition to being
an automatic miss, also indicates the weapon has jammed. A
jammed weapon cannot be fired until it is cleared. Clearing
a jam is a Full Action that requires a Ballistic Skill Test. If
the Test is successful then the jam has been cleared, though
the weapon needs to be reloaded and any ammo in it is lost.
If the Test is failed, the weapon is still jammed, though the
character can attempt to clear it again next Round.

QUICK-RELEASE
The mechanism for removing a magazine is carefully
modified so that it may be ejected forcibly and rapidly. As
a consequence of these changes, the weapon’s Reload time
is reduced by a Half Action, down to a minimum of a Half
Action. For example, a weapon that normally took 2 Full
Actions to reload may be reloaded with one Full and one Half
Action. However, any time the character suffers a Jam result,
the weapon’s magazine also ejects and must be reloaded.

Since the magazine is lost anyway, Quick-Release right now has no drawback. Whether that is a bad thing is another question, but it seems to me as if it's not intended this way. Considering how crippling the loss of high-quality ammo that you only have small amounts of can be, perhaps it would be best to strike the italicized sentence from the Weapon Jams section?

Losing the ammo on a jam has been the case for the previous four systems, so I'm not sure why you'd remove that. Also not sure why there needs to be a downside for a weapons upgrade, since (as I see it) that would make the idea of getting such an upgrade much less attractive.

Its technically a customization, and its meaningful to notice that all the customization options have drawbacks. This one in particular though, does not, because the jam mechanics are wonky (losing all the ammo is a cop out).

@HTMC

As Kommissar noted, this is not a weapon upgrade, but a customization. It's supposed to have at least a small downside as well.

Also, I know that it has been the rule for the previous games. However, consider the Ammo Backpack: 200 rounds of SP ammunition (which are considered one clip), all irrevocably lost on a 96-00? Not that losing a month's salary in bolts was any better in Dark Heresy…

I am not at all familiar with real-life gun operation - and I do not presume to assert that the WH40k roleplaying line should start embracing realism in all its mechanics - but doesn't a jam just result in the loss of a single round? That seems like it would be a good baseline mechanic for the "regular jam": it's still inconvenient but, as Cifer pointed out in their examples, not going to result in you losing the entirety of those oh-so-precious rounds simply because you had the misfortune to roll a 96 to 100. I know it is easier to change a specific weapon upgrade than a baseline game mechanic, but I feel there should be a re-evaluation of punishment for poor rolling.

That, and or requiring you to reload the same mag, instead of losing the ammo entirely.

Or it could be an effect from different weapon types (SP vs Energy).