Rules state nothing remains on the map between stages - but what about Valor and Elixir tokens on the hero's have collected. In campaigns, they removed at end of quests but not sure about stages in Delve.
At the end of any round, when the portal is open,
messages may display in the app asking if all heroes
are on or adjacent to the portal and if they wish
to depart. If “Yes” is selected to both, the entire map is cleared, and all
heroes, monsters, tiles, and tokens are removed in preparation for the
upgrade phase and the next stage.
Note: Nothing remains on the map in between stages, including familiars
and special class tokens. The only exception relates to Challara, whose
familiar, “Brightblaze,” is placed adjacent to her when the heroes are
placed on the map of a new stage.
“Don't try the paranormal until you know what's normal.” - Granny Weatherwax
I see the various stages of the Delve mode as a series of encounters, and apply the same rules that would bear in 2-encounters quests of the "normal" campaign mode.
The Road to Legend's rules refer to class tokens on the map (Traps, Images). They would be discard between encounters in campaign mode.
Elixir, Valor and Vision tokens are kept on the hero sheets, and I believe they persist between encounters even in Delve mode (they surely do in the normal campaign mode).
Hex tokens and the Tracking token mark monsters, so they will be discarded when the monsters are removed from the map at the end of an encounter.
Song tokens return to the Bard's reserve at the end of an encounter. I believe they also do in Delve mode.