Wifi Icons (Airplane Mode Icon, Wifi Icon) often doesn't display. I am using default stock icons. Sometime they appear and then disappear sometime after. Reproducible: Couldn't Reproduce
What plasma-nm version do you use? What do you mean by disappearing? Only icons will disappear or completely icons with checkboxes? If only icons and you still see small boxes, then there is an issue with your plasma theme (probably), if icons disappear together with checkboxes then there might be issue with plasma-nm.
I am using Plasma 5.4.2. I am using stock icon theme. Only icons disappear.
Do you really use Breeze or Breeze dark plasma theme? Note that plasma-nm icons are taken from Plasma theme and not from system icons and Breeze is the only theme with complete icon set. Is that a fresh installation with a fresh user or you upgraded from some older version of Plasma 5 or even from KDE 4?
I use Breeze. I upgraded from Plasma 5.4.1 to 5.4.2.
I would recommend trying this with a new user or at least try to kill plasmashell, clear plasma cache (in $HOME/.cache), run kbuildsycoca5 and then start plasmashell again. Anyway, this doesn't look like a bug in plasma-nm.
*** Bug 368660 has been marked as a duplicate of this bug. ***
This is fixed now, by improvements in the icon theme.