Your other bamboos are also all hardy to -25C whereas the Him..mus is only hardy to about -8C. Your Hf is a bigger warmth-junkie than your other bamboos. It needs more warmth to thrive than we have been having recently. Lots of other warmth-loving plants have been just the same this year. My outdoor chilli peppers and squash are weeks behind on where they have been any previous year I have been growing them. People growing bananas are saying similar things.

Like you, several of my bamboos have loved this wet, if cool, June and are putting out their biggest ever culms this year. But another one is sulking; no new culms at all from it for the second year running. There's a sulk for you.
It's a very hungry boo and I haven't been giving it a summer feed; it's a very thirsty boo and all I do is give it the odd bucket-full during extended dry spells. These are things I can fix. Harder to fix the weather.

You may find, if we have a warm July, that it shoots late. But then you'll be at risk that the culms don't harden off in time to survive the winter: that is a common problem with its close relative candy-cane bamboo (Damarapa). I've also learned to snip off any late shootings (it's been known to try shooting as late as end September) on my Thamnocalamus as they never harden off in time, and it is just a waste of energy.