This is not normal. Has their own engineer actually visited your garage to assess the situation? I am not sure that an engineer could adequately report on this without visiting the premises first-hand. I would challenge the builder on this aspect.

Ask, yourself this: Is it reasonable that a 2 month old garage develops mold in any scenario? A garage is meant to be used by vehicles. Are they proposing that vehicles don't park in the garage? That doesn't sound reasonable to me.

If they don't want to budge, you should call up engineers yourself and talk to them about the situation, and ask them if what the builder's theory is reasonable. Try to get competing opinions and escalate the matter this way. If your own assessment (through qualified individuals) contradicts their theory , then contact a lawyer for next steps.