I’ve had some of my tubeless tires act similarly. As long as they hold air for a 2-3 hour ride, I usually tolerate it. It’s when they go completely flat overnight that I start over, figure out what’s wrong, and get them sealed better.

Putting it in water, you might find it’s leaking around the stem. In that case it’s easy to assume the nut needs to be tighter, but that’s not always the case. I find it sometimes helps to let the air out, loosen the nut, wiggle the stem around a little or twist it to a new position (unless it’s the kind with a spacer shaped to match the rim contour), and then tighten the nut again—just snug. Not too tight!

Of course, this all assumes you have enough sealant inside and it’s not dried up. Have you checked that?