Morrissey at the moment seems to lack any management beyond his tour manager, his PA and Sam.
His promoter is SJM who run some pretty big shows and artists tours, including all of Morrissey’s for many years now. They’d of likely made the posters and then passed on to ‘M’s Team’ to post on socials and promote.
I can absolutely guarantee there is probably an email out there saying
Provisional tour poster for upcoming UK summer dates. We are awaiting to confirm final logistics with: (insert: PA equipment / venue / etc etc) and will share confirmation when this is all water tight. Please do not share externally“
Then this probably got fully ignored and the date posted.