The Throne of Eldraine cards aren't marked as being legal in Modern.

Example: All That Glitters

deckbox: https://deckbox.org/mtg/All%20That%20Gl … ting=46058
"Formats    Legal in Standard, Vintage, Legacy"

gatherer: https://gatherer.wizards.com/Pages/Card … eid=472964
"Modern    Legal"

+1

MTG Arena wants a particular format to import decks. Please add an export option compatible with the format it wants.