Since only 2 heads are affected, im not so sure it would be an alkalinity/water quality issue. I think it would more likely be something like @nereefpat said, where it only affected 2 of the heads. They do like low flow, so some of the heads being in high flow would definitely do that. If you have coral revive or other coral cleansers/dips, you could try dipping it to see if that helps.
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