Not able to use custom frame rate during conversion of MP4 / H.264

The issue is with converting video. Under profile edition. When I try to convert MP4 to a different framerate - I can increase or decrease the framerate with the pushbuttons up/down. OR keep it same as the source. But if I change it manually - it will save the setting, but it's ignored during conversion. A typical frame rateused is 23.98 or 29.27. If I try to use a frame rate like that - it is just ignored during conversion, and the effect is like having used "Same as source". My VLC version is 3.0.16