Hi,
I tested this out with a few random arcs and am not seeing this happen, however it sounds like there must be an issue for some specific cases.
After the arc disappears, check the History of the three arc points to see if the coordinate has changed. Basically, if the two distances from the radial point to each endpoint becomes different then the program will decide that the arc is not valid and therefore will remove it from the drawing. If the coordinates don't change, the arc will/should remain.
If you have a drawing where this happens, please email me the .dwg and .msz files to brian.sloman@microsurvey.com and I will check it out. Just include a note to say which point id's make up the disappearing arc. Also please confirm whether you are using 2010 or some older version.
fish's suggestion to remove the arc from the database prior to re-importing the points is a good workaround, as long as you know which arc(s) will disappear.