The answer to your question has been posted 3 times in the last 50 posts. But to sum it up for you, GPU has overhead loaded into it. Either put a 6 or 8GB card as GPU0 or use on-cpu video.
I get the problem even when disabling the first GPU. The overhead is a poor excuse for a miner that is simply outdated. His Zcash miner is even more outdated.Are you bitching in his thread that he's abandoned for almost a year that he's not supporting the software? Several people have already mentioned his last post date. He's still raking in thousands of dollars every day so "poor excuse of a miner" when over 50% of casual miners were using it at times seems like sour grapes.
The miner works fine when a 6 or 8GB card is placed in GPU0 position. Anything card that doesn't work past that, well that's the whole point of a DAG loaded into memory or Dagger Hashimoto would have been dead way back in 2017 with ASICs.
Go try Phoenix miner.
6GB or 8GB card may be in any position, no need just to be GPU0. I have several rigs, with random 8GB card placement between 4GB cards and all rigs are working fine.