When the 20th Century Fox logo fades away, the X in the logo stays for a second longer before it also fades away.
The X in the 20th Century Fox logo and the O in the film title are on fire.
SPOILER: The X-Films' title sequences features a weapon used by the villain of each film, backed by the X-Men's Cerebro device.
In this film the title appears with flames, without Cerebro. This reflects the Phoenix being the film's antagonist. Furthermore, Charles had previously released the Phoenix in X-Men: Apocalypse (2016) through a Cerebro door.