Making History Triangular Structural Panels Mirror Symmetry Issue
Mirror symmetry mode for right-angled structural panels of the Making History expansion does not mirror part rotation properly, i.e. similar to triangular lifting surfaces. Instead the part rotation seems to be set according to radial symmetry mode, as shown in the attached images. This issue exists in all versions since the release of Making History.
This behaviour might be intended. If it is, consider this post feedback to possibly change it, since the current behaviour clashes with expectations established by other parts.
I can confirm I have this same problem, and have had it since Making History first came out. The default orientation and mirroring of these parts makes them nearly useless for their intended purpose, seeing as how the only way to make them orient properly is to attach them one at a time and manually try to make them symmetrical by eye, which doesn't really work either.