Cause of saccharine like aftertaste

[CHBKorea]

So I brewed an extract kit, fermented for a couple weeks and bottled it. The extract was old (cleaned the dust and cobwebs from the box), and flavors are off-ish (ok honestly it's beer, but just). The kit claimed to be a Canadian Ale so is a bit malty with a hint of the old hops.

Now, I'm trying to figure out the saccharine after-taste I get from the beer. I get that my ingedient selection sucked and will fix that, but I don't want a glaring oversight of process to give me the same result.

Beer was brewed with 2.5 gallons of distilled water, added half-extract, and did a half-extract boil with proper hops additions. Last 5, added remaining extracts, whirlfloc. Cooled, moved to fermenter and topped with distilled. Pitched vendor's yeast (a screw-up, but hey was my second 5 gallon batch).

Is this just old LME/DME; stale hops; crappy yeast? Is my brew kettle leaching lead into the beer? Help?

I looked around and can't find a source of a "saccharine" aftertaste, but seriously this is like drinking tab in the 70's (except this does the drunk job admirably, so I guess more like drinking Tab+Vodka).

Thanks in advance for any help or insight!

[NervousDad]

Was is liquid yeast or dry? I'd say you pitched a unhealthy amount of yeast and it's under attenuated.

Did you take a final gravity reading? Sounds like fusel alcohols and a high FG.

[CHBKorea]

Was the Munton's dry that came with the kit; 11.0P (1.043) OG, 5.0P FG (1.005 adjusted?) - I calc that out to ~5.0-5.5 ABV give or take. Fermentation on the batch stayed between 64-70F.

Really kind of at a loss, the off aftertaste is subtle and hits after everything else, but lingers.