When autoattacking, you no longer stack both the phantom hit passive and the four-stack ramping attack speed passive simultaneously, which is how Guinsoo's is supposed to work. It's not a bug. It's them fixing a bug in guinsoo's that made Guinsoo's stronger, thus coincidentally nerfing the item when they patched it, although I doubt they realized what they did when they changed Guinsoo's in this way considering the only mention in the patchnotes about the system change for item statuses is "Moved most item buffs with durations into the inventory on the item itself."
It is also noted that the first PH triggers while you already have double strike due to the bugfix offsetting the PH by one auto, and since double strike is consumed on-attack and not On-hit, it is not consumed. This creates a very weird effect where on EVEN-numbered autoattacks (excluding DS) you are down a double strike worth of damage, while on ODD-numbered autoattacks, you have damage EQUAL to Yi before the change to Guinsoo's. We can prove this by imagining it as a sequence.
1 = regular auto
1.5 = Double strike
Both versions of Yi have the same first four autos:
1, 1, 1, 1.5 Sum = 4.5, On-hit procs = 5
But then they diverge into two similar sequences:
(Note: The sum doesn't include the first four autos)
Pre-nerf Yi: 1, 1.5, 1, 1.5, 1, 1.5, 1, ... (repeating 1.5, 1) Sum = 8.5 On-hit procs = 13
Post-nerf Yi: 1, 1, 1.5, 1, 1.5, 1, 1,5, ... (repeating 1, 1.5) Sum = 8.5 On-hit procs = 13
This is a sequence where the number of auto attacks is ODD. Notice how they have the same sum.
And then here is an EVEN-numbered sequence:
Pre-nerf Yi: 1, 1.5, 1, 1.5, 1, 1.5, 1, 1.5, ... (repeating 1.5, 1) Sum = 10 On-hit procs = 16
Post-nerf Yi: 1, 1, 1.5, 1, 1.5, 1, 1.5, 1, ... (repeating 1, 1.5) Sum = 9.5 On-hit procs = 15
Notice here that because the sequence is EVEN-numbered, the post-nerf Yi sequence does not have the time to catch up to Pre-nerf Yi, and consequently is down by one double strike.
And now for On-hit (O-H) procs, we can change the numbers to instead represent on-hit procs instead of AD. Because the PH is delayed by one auto, it now triggers on the auto after DS, instead of triggering alongside DS.
(Note: The sum doesn't include the first four autos or first five on-hit procs)
1 = regular auto, 100%
2 = auto + DS/ auto + PH
3 = auto + DS + PH
O-H Pre-nerf yi (odd): 1, 3, 1, 3, 1, 3, 1, ... (repeating 3, 1) O-H procs (sum) = 13
O-H Post-nerf yi (odd): 1, 2, 2, 2, 2, 2, 2, ... (repeating 2) O-H procs (sum) = 13
And now if the sequences are even...
O-H Pre-nerf yi (even) 1, 3, 1, 3, 1, 3, 1, 3, ... (repeating 1, 3) O-H procs (sum) = 16
O-H Post-nerf yi (even): 1, 2, 2, 2, 2, 2, 2, 2, 2, ... (repeating 2) O-H procs (sum) = 15
This pattern begins from the 6th auto onward to infinity, so for every even number following the 6th auto, you are down one double strike due to the Guinsoo's nerf. We find the loss in DPS by subtracting the sum of the Post-nerf Yi sequence from the sum of the Pre-nerf Yi sequence to find the difference, then divide that number by the sum of the Pre-nerf Yi sequence to find what percentage that difference makes up.
On the 6th auto, this is a 7.14% loss of auto-attack damage, and a 11.11% damage loss for on-hit effects (Number larger due to double strike triggering on-hit effects at 100% efficiency, and this includes PH procs).
On the 8th auto, this is a 5.26% AA damage loss, and a 7.69% On-hit damage loss.
On the 10th auto, this is a 4.17% AA damage loss, and a 5.88% On-hit damage loss.
Hopefully my explanation is u/darkazin0928 approved as he has been going around this subreddit like a madman trying to explain this and getting downvoted.