“I want my rights and I want my career back,” Joslin wrote in the post.
The NPS confirmed that several employees were facing administrative action “for failing to follow National Park Service regulations”.
A spokesperson said there were “possible criminal charges against several park visitors who are alleged to have violated federal laws and regulations related to demonstrations”.
The Trump administration has introduced a host of policies attacking transgender rights since his second term began in January.
Joslin told The Associated Press they hung the flag “because there were a lot of policies coming from the current administration that target trans people”.
They were inspired by Trump’s executive order banning transgender competitors from women’s sports.
The flag was meant to communicate that “we’re all safe in national parks”.