Blank page number when copying/pasting sheets

Description

When pasting a hierarchical sheet instance, the page number information is removed (it is blank instead). This in turn results in the page number not being editable (trying to change the page number when originally it was blank has no effect).

Steps to reproduce

1. Open the Schematic Editor
2. Add a Hierarchical sheet (Hotkey S)
3. Call the sheet "sheet 1"
4. Select the hierarchical sheet and copy it (Ctrl + C)
5. Paste (Ctrl + V) the copied hierarchical sheet
6. Open the properties (Hotkey E) of the newly pasted sheet and note that the page number is blank (screenshot below).
7. Attempt to change the page number (e.g. page "7"), press "OK".
8. Open the properties (Hotkey E) again and note that the page number is still blank.

KiCad Version

Application: KiCad Schematic Editor

Version: (5.99.0-9713-g2bc22de1a5), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.71.0 OpenSSL/1.1.1g (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.41.0

Platform: Windows 10 (build 18363), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Mar 11 2021 10:00:57
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.73.0
	OCE: 6.9.1
	Curl: 7.71.0
	ngspice: 34
	Compiler: GCC 10.2.0 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON