Now I understand why.  A very strong argument for upgrading the weatherflaps and mudflaps to wool in future.

(This picture is the weatherflap on one of the doors, sewn to the door.)

They aren't now because I ran out of the main fabric, and as they needn't match, or be light-colored, I'll find something I like in future based on weight and weave type.

The fine print:  After my rain test Friday night,  I let the tent dry all day Saturday.  "Dry" is a relative term in Florida.  I put the fairly dry tent in my car that night, intending to put it back up again the next day at a shire meeting.  But then mundane stuff got in the way, and I brought the tent inside.  That was Sunday.  Today is Tuesday.  Man, that stuff is fast.

I won't try using bleach to deal with the mildew, because chlorine bleach dissolves wool (and linen – I have some very nice holes in the cuffs of some office trousers from a pool deck).  I think if I decide to do anything, it'll be ammonia…but more likely, I'll replace the cotton with wool before I use the tent again.

Bright spot, it does look more broken-in now.