1. Fatigue:

As you know, Ultraman Noa always travelling the dimensions and universe, because Ultraman Noa always travelling dimensions and the universe, so he needs hibernation once every 1000 years, due to fatigue and wanting to start hibernation, in that moments Etelgar takes advantage to seal Ultraman Noa who fatigue and forced Noa to downgraded become Ultraman Nexus.

2. Eteldummy & Etelgar Castile:

as you know, Etelgar's castle can travel space and time and also Eteldummy is a picture of the fear of an Ultraman, the Eteldummy that Ultraman Noa (Nexus) fought is Eteldummy Dark Mephisto where Dark Mephisto is Himeya's biggest fear, plus the quotes used by Nexus when fighting Eteldummy Dark Mephisto is quotes from Jun Himeya says "Light is a bond, Whoever dims it, it will shine again", This is further strengthened because the forms used by Ultraman Nexus are Anphans & Junis.

If it's true that is Ultraman Noa, then the Eteldummy that Ultraman Noa faced is Eteldummy Dark Zagi or not Dark Lucifer,

so there is a possibility that Etelgar sealed Ultraman Noa (Nexus) when the timeline was still in Jun Himeya's arch.

And again this is just a theory.