When creating a HDR from manually exposed source images, the "Exposure" value is "+0.00 EV" for all of them. It is possible to adjust the exposure using the slider below the preview, but this requires manually calculating what the actual exposure difference was. When combined with autoalign, the resulting HDR is messed up as well, looking much the same as naively leaving the exposure values at zero.
FWIW I use manual exposure frequently to create HDRs when the range provided by my camera's exposure compensation isn't large enough. For example, combining full sun and deep shadow can easily require -4EV compensation or more to contain the bright highlights.