I would ask the HOA to get an inspection done by a professional to confirm their assumption that it is a foundation issue and not a stucco issue. That way, the HOA can substantiate their claim and maybe even discover from the professional they hire that it is, in fact, a problem with the exterior stucco. From there, if the exterior stucco is the HOA’s responsibility, then the HOA should fix the problem. If the water damage is severe and you cannot wait any longer, you can hire a company to fix it and then have the HOA reimburse you for the expense later on.
Hope this helps.