I also encountered this same issue when accessing my VCSA 6.5.0.5100 through the vSphere (Flash) web client through 64-bit FireFox 64.0
I simply needed to rename the VMs visible name in the VM inventory and when accessing the flash client I couldn't erase/modify the field using my keyboard, it's like any key I hit did not register or do anything. I have encountered this before in other fields, finally decided to look into it.
I was able to modify the VM names through the HTML5 vSphere Web client. When accessing the vCenter it does say that the HTML5 client has "partial functionality" I am not sure what can/can't be seen/done; however, using the HTML5 client may be a workaround for those using vCenter 6.5 or later. This was my first time accessing the HTML5 client and I might consider switching to using it primarily.