This is a known issue which may happen if the plugin is internally loaded via built-in bit-bridge of FL Studio. To overcome the issue make sure to install and use the 64-bit version of the plugin.
This is not related to Voxengo plugins, but to the way FL Studio handles multi-input VST2 plugins: it applies some "makeup gain". You may switch to using VST3 versions of plugins: this usually fixes the issue.