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I honestly didn't carefully monitor it. I just noticed it looked that way when it was facing away from me. It is new hair under there, so I imagine it all grows in at the same rate. Brachypelmas don't molt very often so be prepared for a long wait. There really isn't any reliable way to predict...
My B auratum also came with a bald butt. It just means they were kicking hairs. Who knows why at this point. That bald spot will darken up before they molt. Here is mine's butt a week or two before they molted.
Grammostola acteon. I had no idea she was going to molt. I gave her a dubia roach. I checked the next morning and she had built a molt mat over the roach. She was on her back so I didn't dare try to take it out. The roach is still webbed down! I want to take credit for documenting a tarantula...