My business model may be different than yours. I work directly with all my clients, no middle person or AMC. My clients hire me to provide them with an appraisal. They will provide me with the information they have and then expect me to chase down anything else I believe is necessary. This looks like it would be one of those situations. It appears the exact legal description was not available when the appraisal was ordered. I would contact the those most involved in requesting the split and go from there. I would also send my client an email explaining what issues(s) I was having and what steps I had taken to correct it. Included in that email would be an estimate of the additional time required to complete the report and depending on what information I don't have if they want me to move ahead without it. You would be surprised how excited someone gets when the lender calls and says your request is dead in the water until the appraiser receives this information.
Yes, this does require some time and effort on my part. My fees are not extravagant, but they are set so that a couple of hours of time for unexpected work is figured in. When asked to bid, which is only about 50% of the time, I provide the client with a not to exceed bid. Many times, my final invoice will be slightly below the not to exceed bid. If my time runs over the not to exceed bid, then I eat it. May not work for others, but my clients know that once I provide them with a bid, I will not be coming back to them and say the project was more time consuming than expected and I need to be paid more.
A quick and recent example. I received an assignment, a couple of weeks ago, from an out of the area client for an appraisal of a to be constructed dollar type store. While there was a lot of documentation provided, from a location standpoint all I received was the name of the County, the seller and the buyer. The seller owned multiple parcels, so I searched Planning Commission minutes and found a request for a parcel split involving the developer which included the proposed legal description. Probably took about 15 minutes of my time, which is less time than it would have taken to throw a hissy fit and demand the lender get me the information.
One always has the choice of being a provider of professional services or an order taker.