Switch from vent to defrost?

1995 zj jeep grand cherokee 4.0l

Does any one have an idea as to why ? when I the heats on vent during acceleration it switches to defrost and blows air until to smooth out the acceleration…?

Is it a vacuum line some where ?

It’s probably the one way check valve in the vacuum line between the engine and the AC controls.

Yeah, it’s probably a vacuum problem