Heatlight stress?

600ppfd over 20h is 42+ DLI. probably too much. But if using a phone app, might be within the +/- 10% error and could be fine if it's the minus 10% error. phones are not quantum meters. all a ppf/ppe reading does (not ppfd because that is per m^2 and not a single point measurement) is convert a klux reading using a one-size-fits-all conversion factor that will not account for differing CCT and other properties that might vary from Lux/lumens - hence up to 10% or more error +/- on readings from a phone app. it's a consistent value, which can be useful, but don't take it as accurate. it is precise, which is useful. So, if "40" dli has teh resulting growth patterns you want, then that is a good value to shoot for. if 45 is, then so bet it, but it probably isn't 45DLI as that simply doesn't jive with ambient CO2 levels. plant may adapt to it but growth rate willnot increase with it above what the ambient CO2 allows. 1 day won't typically cause huge problems, so just as likley nute-related. balance being off a bit. watch progression. compare to a leaf symptom chart. do your best to eliminate multiple possibiities. this stuff is not discrete. it's almost always a guess and you have more info at your hands than we do reading this question. trust yourself. trust the leaf chart. be consistent in how you do things.