I haven't perused this beta myself yet, but aren't the nebulas used here the new nebulas? These could be the source of the problem (I do recall that the new nebulas do have this problem now and then, although I think these glitches are outweighed by the awesomeness of the new nebulas...).
AFAIK, the nebula is hardcoded in a strange way, obligating formats (PCX?) that are not at all amusing to code around and Zacam was pretty frustrated with this issue.
Until some Valathil - level genius comes around and "fixes" the code for the nebula for good, we will have always strange glitches, etc., IMHO.