Red onions going to mush

A few of my spring planted Red Baron sets have turned to mush.  They made reasonable growth but a handful here and there developed yellowing leaves and when tugged came away from the soil.   There is no sign of any maggots, pupae or insects or any white mould or black specks, but the bulbs are mushy with a vinegary smell.   The Stuttgarter Giant and Yellow Rynsburger at the other end of the same bed are unaffected as are the rest of the Red Barons.   This is the second year I have cultivated in this raised bed, last year it had brassicas.  Before that there has been nothing growing for at least 6 years as we have only just reclaimed this part of the plot.

Any ideas what is wrong?  Symptoms don't seem to fit usual onion problems.

No white fluffy stuff.  No black dots.

I've lost all my red onions this year and it looks as of that was  the problem. My whites are doing well at the moment.