NNJoin in QGIS does not return any distance

I have two layers, one with ~100,000 points, and another with ~20 polygons.

I’m trying to find:
a. which polygon a point belongs to
b. the distance from the point to that polygon

I understand that NNJoin should be able to do both of these. However, it only gives me the answer for (a). The "distance" column – which should contain the answer to (b) – is totally filled with 0s.

When I run NNJoin, I get a warning message saying "Geographic CRS used for the join layer – distances will be in decimal degrees" – but I get 0s, not decimal degrees. I’ve checked that I’m using the same CRS throughout my project (EPSG:4326), so I’m stumped (I get perfect distance values, in metres, using the "distance to nearest hub" tool). Does anyone know how to fix this?