Missing H1 in post

Hello all,

Please can someone help me work out why the h1 on this page is missing? I have set up the same format as per all other posts, but this one isn't functioning correctly

https://reclaimdesign.org/how-to-make-vertical-garden

For reasons unknown to me the post in edit mode has the h1 greyed out, this is the only difference I can see, and I cannot see any way to amend?

https://www.dropbox.com/s/o3rp13pkkrpceo6/screen1.jpg?dl=0

https://www.dropbox.com/s/cjorhxq0bbqn61h/screen2.jpg?dl=0

https://www.dropbox.com/s/zihaityksxf6yhs/screen3.jpg?dl=0

https://www.dropbox.com/s/lc5r1bsee68ulk1/screen4.jpg?dl=0

Many thanks for your help.

Michael.