This could be an example of a failure in the old dynamic lighting system that cropped up last year. It won't be fixed, since the legacy system is deprecated. The best bet is to switch to the new system. The old system is also prone to failure from unclosed lines. If you are indeed using the new lighting system, then ST Carcass has probably correctly identified the issue. In this case it likely comes from how vector joint corners are "mitered". Too sharp a join (and it has to be extremely sharp, like doubling back on itself), can cause unintended results. For this reason, it is also a good idea not to use the freehand tool for dynamic lighting. The kajillion short segments produced by that tool can inadvertently create corners like these.