I set up the folowing:

Large Cargo [TIM Ice:P1]
OxyGen [TIM EXEMPT Ice:P2:20]
Ref [TIM FORCE Ore/Ice: P4] (fyi, block is disabled)
All blocks are empty

I drop 100 units of ice into Large Cargo. I would expect 20 to go to Oxygen and none to the Ref because it is a lower priority than Large Cargo. While OxyGen gets its 20, Ref gets 80, 0 is left in Largo Cargo.

I also noticed the following

OxyGen1 [TIM Exempt Ice:P2:20]
OxyGen2 [TIM Exempt Ice:P3:20]

Both blocks are empty

I drop 40 ice in an untagged cargo container and each OxyGen gets 20 as expected.
I change both to :30 and drop in 20 more ice, expecting each to get 10 more.
Only the P2:30 gets an additional 10 units of Ice. The P3:30 gets none. There are no other requests for Ice on the system.
If I empty out both OxyGens and drop in 60 units of ice then they each will get 30.