I can replicate this - both by changing the Color Management 'Exposure' setting or by changing the Film 'Exposure' setting - although the two seem to be scaled differently, with the -10 -> +10 Color Management setting changing by a factor of 2 for each unit change (so '0' is 1, '1' is 2, '2' is 4, '3' is 8, etc. - with similar for negative values, so '-9' in your example seems to actually relate to a 'Film' exposure of 1/2^9 = 1/512 - so negative values are actually valid in this case.
After varying various other factors, I managed to get around the 'darkening' by disabling the 'Indirect Light' setting in the Light Paths panel (at 2.8 - at 2.79 this was in the Sampling panel). This seems to indicate that the issue is that the Indirect Light is being lost due to being below the Indirect Light clamping setting. Setting Indirect Light clamping to zero (to disable clamping) fixes the issue in my case.