Phaph Fumu’s Delight, flower bud turned brown

[smenard]

I was very excited when I saw the sheet of a new flower bud emerged few weeks ago.
Unfortunately it stop developing and turned brown.
My plant is potted in bark with a thin layer of sphagnum moss. I water it once a week with tap water, fertilise it once a month during winter period, and mg or calcium once a month.
Any idea of what happened?

I have search OB without success