I'm pretty sure I know what's going on here. You should see the bug
if the "Include Refraction" checkbox is ticked in the Locations dialog,
and one of the two points is below the horizon. If the box isn't ticked,
or if both points are above the horizon, everything should work.

A while back, I was asked about whether the angular measurement was for
the refracted positions, or the airless ones. At the time, it was for the
airless positions, so I added some code so that the refracted positions
would be used, if the "Include Refraction" check-box was ticked in the
Locations dialogue.

The problem is that if you ask for the angular separation for a point
below the horizon, you get a meaningless result at best. Or, in general,
if you ask for a refracted position below the horizon, you'll get a
meaningless result. This why, if you have alt/az shown in the legend
area, you'll see a jump as the cursor goes below the horizon: Guide
switches from showing a refracted altitude to an unrefracted one.

I expect to fix this by having Guide show the separation between the
refracted positions if both positions are above the horizon _and_ the
"Include Refraction" check-box is set. Otherwise, unrefracted data
will be given.

For the nonce, one can turn off the "Include Refraction" box when
measuring positions below the horizon.

-- Bill