Problems with Air and Nose

[tjcombo]

@Carsen-Abraham I think I know exactly what you mean and used to suffer the same problem in my first couple of years playing. It's more than 50 years back, but I remember exactly what and when it happened...
At band practice for the local brass band, later in the evening, sitting playing dumpty-dumpty-dumpty dum boring-as-bat-sht third cornet parts, it was like something in the upper pallet just let go and air started come out of my nose. There was no way I could direct the entire air stream to go through the horn. I don't recall ever having the issue whilst practicing. The problem went away after a year or two and I advanced to second cornet and then repiano parts. I don't think that the more interesting parts changed things. It was probably fatigue and tension due to fatigue and poor technique. At the time I was only nine years old and band practice ran a couple of hours past normal bedtime.
Since the advent of Dr Google, I've come to the conclusion that the condition was pharyngeal insufficiency - as nailed by @Dr-GO above.
I have no qualifications to make a diagnoses, but wonder if you have the problem all of the time - from warm-up through normal playing, or just when fatigued? If it's all the time, then seeking a medical opinion would be in order (problem is finding a physician who understands the physiology of playing brass). Otherwise, get a teacher who can make an assessment of how you are using your body WRT to tension and air-use - if all is in order, then you're back to seeking medical advice.

good luck!