Hi,
You set your regulator very high, is it really necessary?
With my huben .25, regulator set at 160 bars with 35 clicks and zan 38gr .253 I get 984 fps, but it shoots better at 944 fps 32 clicks (it groups in 3cm at 60 meter).
I had a Huben 22 in 2018 with huben slugs 40 gr I had problems like yours, the slugs too heavy and / or tight creates too much back pressure on the valve,It even breaks the aluminium plate behind the valve.
For me, using 50-grain slugs is not advisable on a hammerless system.
The old huben .22, 40 gr slugs looked exactly like the Altaros, but I think there may also be a issue with the shape of the slug and how the seal is made in the magazine, A “cup base” or "flat base" helping the slug to come out faster.