Forward movement stops when mounted

[Lord Warfin.1209]

The first time I start moving forward after mounting my character moves a short distance and then stops even though I'm still holding down the forward key. If I let the key up and press it again I will start moving again and usually stay moving. Occasionally it does happen more than the first time and even after I've been mounted and moving around for a while but it's not as common. It always happens the first time. I don't think it's the system or my keyboard since it never happens on foot and it's usually just the first time I try to move when mounted. Doesn't matter whether it's the raptor, springer, skimmer or jackal, which are the four I have. This isn't a critical bug since I know how to work around it, but it is annoying. I'm using the Mac client under OS 10.14.6.