Normal in some circumstances, but not desirable. How is the drainage at the location? What's under the mulch, just plain soil? Anything else beside the mulch? Deep litter favors having other organic matter mixed in, to compost down with the poop.
How many birds?
How big is run, in feet by feet?
Dimensions and pics would surely help here.
Oh, and.... Welcome to BYC!@Cheridon
Where in this world are you located?
Climate, and time of year, is almost always a factor.
Please add your general geographical location to your profile.
It's easy to do, and then it's always there!
When you put your draft barriers up, how much ventilation space did you leave and where is that ventilation located?
What's the groundwater and drainage situation? What's the weather been like?
How deep is the mulch? Is it loose and fluffy or has it packed and matted?
With the description of "rotten cabbage" my first instinct is to think that the bedding has gotten both wet and packed so that anaerobic pockets have formed.
Preventing packing/matting is why I favor a mix of bedding materials with different compositions and textures -- wood chips, wood shavings, straw, pine straw, leaves, etc.
Throwing a handful or two of scratch into the bedding a couple times a week enlists the power of chicken labor to mix and turn the bedding, but it still may be necessary to get in there with a pitchfork to stir and fluff the trouble spots (under perches, at the end of the ramp, near the feeder, etc).