For anyone else that finds themselves with this problem, the answer did lie in the projection. If I create a polygon feature layer in AGOL, it will have the WGS 84 coordinate system and give wonky area calculations. To solve this, I opened the feature layer in ArcPro, assigned a State Plane coordinate system, and calculated the area in ArcPro in the layer's attribute table. To check my work, I opened the feature layer in AGOL and used the "draw area" tool in a web map to verify the measured area matches the calculated area, and it did. I also created another data column and calculated the areas in AGOL, again to verify the areas are the same.