the problem is that joystick have lost the aceleration movement, in 4.26 you can move slow or fast your character with virtual joystick, in 4.27 the movement is allways fast, you can´t move the joystick like you want, the position is always fast.Not have medium positions.
It sounds like you mapped the virtual joystick in the ProjectSettings->Input section to W/A/S/D Keys in your 4.27 version.
You have to map the virtual joystick, for example, to the gamepad joystick axis. This way you will receive an -1.0f … 1.0f input value which you can use to smoothly control your character / vehicle 
This happens even with test projects offered by unreal, with any new project, you can try this with third person template. I think that is a bug of 4.27.
Yep I believe this is a regression in 4.27. I hope the unreal devs fix it in a point release! People are mentioning this on the unreal slackers channel too Discord
Hey! I wanted to add 4 way movement using mobile joystick, But i only know how to do it for pc inputs… can you tell me how can I use my joystick as w, a, s, d.?