MA2, are older rims, and not as inherently rigid as modern rims. That makes them much more sensitive to variations in spoke tension. They also won't tolerate the kind of tension that modern (heavier) rims do. So you're very limited in how much tension the right side will tolerate, which means you're that much more limited in left side tension (roughly 60% of right side tension.

If you're building with 2mm plain gauge spokes, you probably have too much spoke for the rim, and the left is under tensioned. The constant realignment may be due to left nipples loosening, or the rim itself settling under load at the right spoke holes. You might get it to settle in, or you may be dealing with this nonsense forever. If it improves, and seems to settle in, you might stabilize the left side by using some wicking threadlocker (Loctite green) between the nipples and rim. Apply from the hub side, and spin the wheel to drive it in. Don't loctite the spoke threads.