I found it to be the same in the Samsung Tizen and XBox apps (which I use on a daily basis) and in those Plex have bizarrely mapped fast forward 10 minutes and rewind 10 minutes to the Channel Up/Down on my Samsung Remotes and XBox Media Remote when viewing Live TV.
Yeah it would also be pretty cool and very useful to be able to type in a channel number when viewing Live TV - just like you can from most set-top box DVRs.
There is a feature request for both here that you might like to vote on if you haven’t already 
