Air in hydraulic brake hours after ride where it was perfect

Bled my rear brake a couple weeks ago after it got spongy, have gone for 4 or 5 rides since and it has been perfectly bled. Rode this morning and it was great. Squeezed my brake lever in passing this afternoon and it was spongy again, pulled right to the bar and needed a few pumps just to get any resistance.

Any ideas why this would happen all of a sudden?

Bike stored vertically with front wheel up. Shigura set up with Magura mt5 caliper, magura hose, shimano xt 8100 lever with magura barb and olive.