If a box with items is placed in the world, then the panel and only then the comparator, the comparator does not read out the box.

Thanks for the report! I have plans to rework the way comparator overrides are handled. I should have a fix for this soon.

I experienced inconsistencies with Mekanism tanks after a user reported it to me. I wasn't able to reproduce it with vanilla or PneumaticCraft inventories or tanks, but in any case, my rework should fix it. You will probably find that if you leave and come back, the comparator will start responding to the chest.

Fixed in 1.16.5-1.8.0b.