I've noticed that for all payouts since November 6th 2018 significant portions of my payouts are being withheld, roughly 28% per transaction. I am a host in Europe.
In the transaction history, next to the amounts deducted to those transferred to me, it says "Tax Withholding". After reaching out to Airbnb I was told this was due to the fact that I was registered as being in the US.
I did not submit any additional documentation, they just consulted all the information I had already provided months ago and changed my location accordingly.