Schematic symbol group vanishes when mirror horizontally. Version 5.99

KiCad_Mirror_Issue_07-17-2021a.zip<!--

Using the attached file, this issue has been easily reproduced (maybe symptoms vary somewhat but they are obvious) by others on the KiCad forum.

Do a horizontal mirroring of the small symbol group in the schematic diagram and it disappears. Vertical mirroring works OK but then rotate and the symbols vanish; wires remain.

Application: KiCad Schematic Editor (64-bit)

Version: (5.99.0-11430-ge61b1f03b8), release build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.74.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19043), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jul 14 2021 09:00:10
	wxWidgets: 3.1.5 (wchar_t,STL containers)
	Boost: 1.75.0
	OCC: 7.5.0
	Curl: 7.74.0-DEV
	ngspice: 34
	Compiler: Visual C++ 1928 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON