When copying polylines and circles (and possible other entities) using mirroring, if one mirrors an entity twice (e.g. reflects it once horizontally and once vertically), copying it each time rather than moving it, the entity is copied successfully. However, the copied entity can not be selected at the location it appears. Instead, it must be selected either at the location it was mirrored from, or another location reflected from that. At any rate, it can not be selected where it actually appears to be. If the file is saved and reloaded, the entity may be selected. Other entities may be selected including the entity that the mirrored copy was created from.
I can select them (using circles or polylines).
What version of LibreCAD and operating system are you using?