Apparently not all seals are the same, even if the part number is the same.
I’m not sure why or how but some genuine Brembo/Triumph seals don’t suit some calipers = they don't seal adequately.This strikes me as very strange, I can only surmise tolerances or seal lip design have been changed.
Thankfully my dealer has come across it before with Triumphs of a similar date of manufacture, so they know exactly which piston & seal kit does work but are currently having problems sourcing it for my 2016 Gen 2 = Triumph have them on back order until at least 6 Nov!