Not so long ago Skoda was setting up Octavia IIs (same suspension as Mk V Golfs) with the wrong camber on the rear. This led to excessive wear on the inside edges of the rear tyres and a "serrated" wear pattern on the tread which made a noise very similar to a duff wheel bearing. The solution was to adjust the camber and replace the noisy tyres.
A lump on the sidewall is usually either damage from a pot hole or similar, OR a manufacturing defect. Seeing as you've had two tyres replaced already, chances are the 3rd one has the same fault. Go back to your dealer and ask them to replace it.
Just for our info, what brand/model/size tyres were replaced ?
2017 MY18 Golf R 7.5 Wolfsburg wagon (boring white) delivered 21 Sep 2017, 2008 Octavia vRS wagon 2.0 TFSI 6M (bright yellow), 2006 T5 Transporter van 2.5 TDI 6M (gone but not forgotten).
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