Haxton was deported and banned from the UK in 1919. According to Wikipedia, "Haxton's Home Office file, containing the reason or reasons for his deportation, is sealed until 2019."
As it's now 2020, has anyone taken a look at his file to find out what the official reason was?
As a Maugham fan, I have been searching for the answer to this question as well. It seems that British rules are different from American ones in this regard; just because a sealed Home Office file CAN be unsealed does not mean it HAS been; someone has to enter a motion to unseal it. If any members of this subgroup are British subjects, they might have better luck with that than I (an American citizen).
Pending a definitive answer from that file, the most popular speculation is that Maugham's wife, Syrie, probably used her substantial connections in the Home Office to have him deported. On what specific grounds is anyone's guess, but before the War, Haxton had been arrested and prosecuted for homosexuality (the same law used to jail Oscar Wilde). Although he was acquitted, homosexual acts were still illegal in Britain in 1919, and Haxton was totally unapologetic about his sexual preferences. Regardless of any official "reason", there was certainly no love lost between Haxton and Syrie Maugham.