Greetings! I ran into the exactly same issue today, October 30th 2023, with the same Archon and in that same room. The only difference is that I was playing in Solo mode.
Here is video footage of me trying to interact with the Archon's body floating in the air:
Relevant information:
How can this bug be reproduced?
The steps may not be 100% reliable, but the only thing that seems to be needed for this glitch to happen is to deliver the last hit to Archon Nira while she is floating in the air while not above ground. I repeated the mission to try to replicate the bug, but did not have luck in fighting Nira in that same room again.
Can you provide your build and game settings?
RTX 2060 and an Intel(R) Core(TM) i7-9700KF CPU.
Warframe settings:
Borderless Fullscreen, 1920x1080 resolution, 144Hz refresh rate, Graphic Engine set to Realistic, and every other graphic setting was on High or turned ON.
What kind of squad were you in? Solo, host, client?
I was in a SOLO squad.