I reproduced this in v18.05. Before you adjust the gain filter’s level, move the play head to somewhere in the middle of the clip. If it is at the very beginning or the very end, then it is not applying your change. This is a problem with the introduction of the new simple keyframes for the gain filter. It thinks you are trying to adjust either the starting or ending value. I will work on a fix for it for the next version. It should not do that unless you have actually used the simple keyframes.