I had the exact same experience, twice actually.
It seemed like it kept the 5% that wasnt Corrupted from the loss and just dotted random 1'ish tile holes elsewhere. it pulsed through 4 waves before I could save enough energy to do cataclysm and lay down an extra purifier.
I dont mind losing, a game where you cant lose is boring. However, I do like it to be a struggle with a chance of winning lol
I dont think it is behaving as intended to be honest. The pulse is supposed to get really strong and I get that, but the starting with no uncorrupted area doesnt seem right