The fullwidth forms (the arrow symbol) of vim plugin vim-powerline cannot display correctly. It only show nothing at that position.
You'll have to take this up with powerline, sorry. They're in a tight spot between the option of using unassigned Unicode code points vs. using private-use code points that rxvt-unicode uses for its own purposes by default.
Please see https://github.com/Lokaltog/vim-powerline/issues/128 and https://github.com/Lokaltog/vim-powerline/issues/223
The arrow glyphs are going to be moved to the private use area in the upcoming version of Powerline.
Resolved in the upcoming version of Powerline: powerline/powerline@53fbfe1.
Since the vim-powerline issue tracker has gone away, here are the details:
Old vim-powerline uses unassigned unicode area (e.g. U+2b80), but when mosh calls wcwidth() to determine its character width the function returns -1 (nonprintable) for unassigned ones, thus mosh removes it.
Some other scripts (e.g. oh-my-zsh agnoster theme) still uses old powerline characters. You have to update them to use with mosh.