Sometimes you can have high GH tap water, but virtually no Mg.
Most times, there's enough.
Adding a bit more via say, MgSO4, epsom salts..........is easy to do, given you dosing past experiences, sounds like a Mg issue.
If the plants respond pretty fast to adding Mg, then you know what's up.
Mg deficiencies correct themselves fast, 1-2 days tops.