Sorry to hear this Rhys, its a pity you hadn’t sowed just a few other onion seeds at the same time, say your maincrops, to have a benchmark.
In theory, onion seeds germinate less well in temperatures consistently over 20-25C, because when bulbs drop seed in their second summer, they need to lie dormant in summer heat and then germinate in early autumn, to over winter as small plants.
However when I grew overwintering onions from sowings in the greenhouse in late August, it was often warmer than 20C, and they grew.
I would suspect the seed but can’t be sure, everything else sounds fine.