Appraiser backed out days before closing over adjacent “buildable” lot—what’s going on?

We ran into a weird issue during a home purchase and I’m trying to figure out if this is normal appraisal complexity or something I’m missing.

The property has two adjacent parcels (basically two lots next to each other), but only one contains a dwelling unit. The other is vacant land. They are both half an acre and zoned for the same thing.

The first appraiser started the assignment but ended up backing out. According to the lender, it was because they viewed the vacant parcel as having “highest and best use” as a buildable lot.

We’re now less than a week from closing, and the lender is trying to get a second appraisal rushed through.

What could be going on here? Should I be concerned? The lender assured me that they often do these types of loans and feels the appraiser is just being difficult.