First thing I thought of was, "Was the weather the exact same both days?". By "weather", I mean temp, wind speed and direction, humidity.
Second: Did you set up the rifle the same way both times, i.e. same bags/rest set up?
Third: When you say custom rifle, can you give us some details? Heavy gun? Light gun? Muzzle brake? Suppressor?
Group size is close enough that I don't think it would be the scope mounts. In my experience loose rings or mount will cause groups to open up significantly, not just shift POI.
You say you cleaned it after the first range session. Was that first group shot during that 1st session, or was that after?
Without the above questions being answered, my first instinct is to suggest that you are starting to develop a flinch or are anticipating the shot and tensing up. For a right handed shooter when you tense for the shot, you tend to put pressure downward and to the left of the buttstock, causing the end of the barrel to move up and to the right. Again, that is pure speculation on my part, but I have seen it enough times for it to be a common answer for me when I see the POI shift up and to the right for right handed shooters.
