Variagated Holly gone yellow

No, it doesn't. Variegated plants often throw out plain shoots. Are all the leaves on one or several shoots/branches yellow? If so, that's a bit surprising, as it would be more likely to revert to plain green - as Holly bushes are naturally green. If this happens, cut off the all-green shoot where it grows from. I would be inclined to leave the all-yellow leaves alone, if it were mine. Just watch for dying leaves which would indicate a viral infection.

Don't forget that evergreens still lose leaves, they just do it all the timw instead of in autumn. Your Hollies could just be reacting to moving. Make sure they are well watered. One of our variegated hollies has an all yellow leafed branch, looks a bt odd,but it 's surviving.

I've got some variegated holly and it often sprouts yellow leaves which I quite like. Doesn't harm the plant.

If it had reverted it would have gone green. Yellow leaves on variegated plants are common, no problem. If the whole plant was yellow and stayed always yellow you would be able to register a new ILEX!!!

Yes, common for variegated hollies to produce all yellow leaves and branches here and there, no problem.