Do we ever see the Senshi de-transform?
In episode 45 of the dub, Sailor Moon is forcibly de-transformed at ~11:44. When it does, she "ribbons" back to her civilian clothes.

In episode 108 of the dub, we see Sailor Uranus de-transforms (at ~21:12), willingly, by going nude briefly before she appears in her civilian clothes.

In the manga. it looks like they use the transformation pen to de-transform a la ribbons.

So it seems they typically get surrounded by ribbons again and revert back to their civilian clothes.