I use the Customize the Ribbon dialog
I'm on my own WiFi network
On that network there are at the moment two 64 bit HP laptops with Win10, one has Office 2013, the other 365. The most recent problems are with 365 but I have the feeling 2013 has misbehaved in a similar way (Will check!)
I had both 2013 and 365 on the same system but ran into problems with 2013 updates affecting 365 so I uninstalled 2013.
The custom tabs stay. In fact, the problem is mainly that the custom tabs don't stay where I want them.
The amount of time it takes for them to decide to return to their previous location varies, obviously some kind of triggering event is involved. I haven't been able to find out which event except that a close down + restart of Word has to be included. The changes never take place mid-use. But sometimes a number of close down-restart events are needed before the change happens.
_Lup