To add to what Hudson wrote, what you are seeing and using when the filter is selected is the editor. You need to deselect the filter somehow to turn off editor and see the effect on video. You found one way. Hudson showed you another way. There are more. And yeah not a bug.
Thank you @shotcut and @Hudson555x for very quick responses and good explanations. I see now that I not yet have fully understood keyframes and was using filter trimmering. Looks like I was doing it correctly, but wasn’t aware of the deselect filter functionality - and it wasn’t and still isn’t quite obvious for me that having the filter selected (on trimmered filter) would lead to result that the filter doesn’t have an effect until I deselect.