Layout switch unwanted combination Rshift+Lshift

Ubuntu 18.04.1 LTS
Linux version 4.15.0-33-generic

WHAT I EXPECT TO HAPPEN:
"R-Shift + L-Shift" hotkey combination doesn't change keyboard layout when not configured to do so.

WHAT HAPPENS INSTEAD:
"R-Shift + L-Shift" changes keyboard layout even when not configured to do so.

------------------------------------------------------------------------------

My machine is set up to use "L-Alt + L-Shift" combination for switching keyboard layout between English and Russian. The command I use to configure this combination:

gsettings set org.gnome.desktop.input-sources xkb-options "['grp:lalt_lshift_toggle']"

With this setting Ubuntu also reacts to "R-Shift + L-Shift" combination and switches the layout as well. This is very irritating, since when typing the C code I often need to use macros like MY_MACRO_WITH_UNDERSCORE, where I naturally hit both shift keys when typing a capital letter after an underscore. The issue appears only when the R-Shift is hit before the L-Shift and not the other way around. The issue doesn't appear when I disable my switch combination with:

gsettings set org.gnome.desktop.input-sources xkb-options "['']"

I also disabled default Super+Space and Super+Shift+Space combinations, although that doesn't affect the issue. I didn't set any other keyboard combinations for switching layouts. The issue appears both in the terminal and in GUI applications.