Possibly a cache issue I experienced some time back.
Context; Wanted to quickly swap out the DHCP address range to bring an old device (esphome, part of a smart switch setup) using a static IP in the range of the old router’s network for my wireless lan’s VLAN. Thought I’d temporarily create a new VLAN with DHCP range to match that of the old router, configure the AP to use the new VLAN on the old SSID, reboot the AP, and hope the old device reconnects such that I can flash a newer firmware without static IP.
So I proceed to do so via the iOS app, except I didn’t wait until the network graphs show up (I’m in Singapore, which may have contributed to the delay I’m not sure if it’s faster in other geographically closer locations to Alta Lab’s HQ) which is typically 5-30s after opening the app. Immediately tapped on Network, tapped the Route 10 (my only device), tapped the + icon and created a VLAN with a number I mentally knew wasn’t occupied, and set the DHCP range as described above. After clicking save, however, the whole network goes down and when I get off the wifi (which isn’t working since it was still configured to the previous VLAN) and open the web UI, I noticed that the only VLAN other than the default 1, was the new temporary VLAN I configured minutes ago. All my 5 other VLANs were completely gone.
Was a big hassle to get them reconfigured before the family woke up 
Now when I make any VLAN changes, I do so on the webUI and ensure the graphs load, and all my old VLANs appear, before I make any changes.