Thanks for getting back to me.
The fluid surface and solid surface were both meshed using a minimum size of 0.5 mm, a maximum of 5.0 mm, and a growth rate of 1.2. The difference was that the fluid volume used poly-hexcore and the solid volume was pure tetrahedral.
You mentioned that this was observed before, but I haven't been able to find anything on it. Could you possibly link me to the source?
I also tried creating a conformal pure tetrahedral mesh and exporting the fluid and solid to separate mesh files, loading the fluid domain, converting it to poly while preserving boundary layers (so it should still be conformal), and then appending the solid mesh to the fluid mesh and fusing them together. The fusing was successful and a wall - wall-shadow pair was created, but the analysis failed once iteration began.