It's always been bugged. Sometimes it'll show as spawned and when you get close to the location it'll disappear again and restart the respawn timer.
It's not really a huge deal since it doesn't happen all the time.
Yeah there's always been 2-3 towns where only 3 sigils appear. The 4th is in a location that bugs it and causes it to either disappear instantly or perhaps never appear at all. So the indicator will light up, go dark, and the timer on sigil spawn starts over again. I know Moonglow is one of these towns. Maybe Jhelom. Perhaps....Ocllo? I forget. Been a long time since I found anyone in a VvV zone.
I've never seen that one in Moonglow, Dexxah. It's possible that it only appears properly on some shards.
I've seen it happen in Brit. Mainly glow and occlo and skara.
It's not gamebreaking. We can deal with it