This doesn't happen to me, however, I am able to cancel the healing from sandvich by switching weapons (it doesn't cancel the animation, but does stop the healing) which is something that doesn't happen on valve servers.
Do you have any kind of script, binding, joystick config, or other functionality that does a weapon switch? That might be the interaction.
Also, any installed mod can change almost anything outside its scope of intended effect. For example I make hitsound and voice changer packs that use scripting, but if I wanted to be an ass I could throw in a mouse sensitivity override which would confuse people who installed it.