Empty 2D weighted histograms when integral < 1

Using Gaudi::Accumulators::WeightedHistogram<2> I find that the resulting TH2D object saved to the ROOT file is empty when filled with values that are below some threshold.

The threshold seems to depend on the number of events, and from what I can tell, the histogram is empty when the integral is less than 1.

A minimal reproducer can be found here: admorris/Gaudi@master...admorris-empty-histos

It includes a 1D weighted histogram to show that the problem is specific to 2D (maybe higher dimensions but I haven't tested).

Attached are plots of 2D histograms with 10 bins from 0 to 10, filled over 10 events at coordinate (5.5, 5.5). One is filled with a weight of 0.10001, the other with 0.09999. The latter one is empty.