It's the time of year, they will get better and cheaper soon, hopefully. Always check the stem end,
that's where what you are describing starts. It should be firm and no wetness or mushiness there. But as your experience, even with the best of of inspection, it happens. Fortunately, we get the fresh onions here in the spring, they still have the tops like a green onion but get as big as an orange or sometimes a grapefruit! A local produce stand has fresh red ones, they are soo good!