I noticed something similar after rebooting my unit earlier today. I think it took nearly 15 minutes after the reboot before I was able to see the unit's WiiFi.
I've also noticed the same issue after upgrading the firmware (going from 1.000 to 1.002). 25 minutes after the update, I'm still not seeing the dash cam in my WiFi list.
This morning I checked to see if I could see the Blackvue on my WiFi, and it still isn't there. I unplugged the unit, plugged it back in...waited a few minutes, and checked again. It's still not there.
I checked and am running the latest version of the Blackvue app for iOS (version 3.21).
I was able to connect to the Blackvue, finally, but only after starting the car. (Maybe it doesn't connect after a FW update on standby power?))
I looked at the Firmware Upgrade in the app on my phone, and it lists the Current Firmware as 1.002 for my DR750X Plus.
Below that section of the same screen it shows the Downloaded Firmware as version 1.002. And at the bottom of the screen it says Firmware Upgrade available, and a button that says Upgrade. Does the app not understand that the FW is updated already?
Checking the Blackvue web site shows that version 1.002 is the current firmware version for this camera.
I minor annoyance I've noticed is that, when you connect to the camera's WiFi, the dashcam announces "Direct WiFi connection already in use". And it repeats that about every 30-60 seconds. Not sure what's up with that.