Slight green apple taste: barrel vs bottle

A curious thing has happened, twice now, and I was wondering if anyone has either experienced something similar, or has any suggestions as to a possible why.

I have done two (almost) identical full mash brews in succession, same equipment and, crucially, the same dried yeast - Safale S04. It's a traditional English bitter of about 3.7% ABV. In both cases I have primed the fermented wort with rehydrated malt extract and then split the batch between 12 pint bottles and a barrel to condition. Conditioning for bottle and barrel was controlled at 20 C (68 F) for a week before cooling to 12 C.

Both drink well, bright, nicely carbonated, good balance of flavours except that, from both batches, the barrelled beer has a very slight apple taste - assume the presence of acetaldehyde. It is in the background, kind of on the swallow, if you get my meaning, but is completely absent in the bottled version.

Cleanliness springs to mind; however, I feel that I have been thorough. Aside from cleanliness, could there be any other reason?

[EDIT] In response to @Pepi comment. The barrel is a 25 litre plastic pressure barrel. The malt extract is added to the barrel and then the fermented wort is syphoned in, submerging the tube in the beer. Once the barrel is full (and mixed without agitation), I then use a bottle wand attached to the barrel tap to fill a few bottles. The caps are plain crown corks, nothing special. Finally, I seal the barrel and inject CO2 into it, leave it to stand, before slowly releasing the pressure to expel air, leaving the CO2 behind.