Fascinating: I've had three Left Joy-Con, and compared to the two Right Joy-Con I've got, the Right ones definitely had a stronger rumble than the Left ones — plus they output a slightly different tone than the Left ones when using the 'Find Controllers' option in the new update.
I think I'll try turning Vibration off, clearing the pairings, re-pairing my Joy-Con, and turning Vibration back on; if that fixes things, that'll be great. : D
Edit: HM! As it turns out, that did — absolutely nothing, actually.
But, interestingly, all but one of the four Joy-Con I have left — the Left one that came with my Switch was defective, so I no longer have it — have about the same vibration strength. The Right Joy-Con that came with my Switch is significantly weaker than the two Lefts and the other Right, though. And the Right Joy-Con from the extra box of 'em that I bought… it seems to be the loudest, but not any stronger than the two Lefts. o.O
In conclusion: …I want a replacement for the Right Joy-Con that came with my Switch; its vibration is wimpy compared to the other three Joy-Cons'. >.>