As far as I believe Wells Fargo is the bank that severed ties:
"After Wells Fargo severed ties, Tether won’t name its banks"
"On Dec. 2, Bitfinex released a quarterly report announcing it would no longer serve U.S. customers because it’s too expensive to do business with them. This followed Wells Fargo & Co.’s decision earlier in the year to end its role as a correspondent bank through which customers in the U.S. could send money to Bitfinex and Tether’s banks in Taiwan. Bitfinex and Tether filed suit against Wells Fargo, but later withdrew the case."
"However, Ronn Torossian, a spokesman for Bitfinex and Tether, refused to identify their current banks unless a reporter signed a non-disclosure agreement, an offer that wasn’t accepted."
