This thread caught my eye because I saw the same initial behavior on my system.
I'll preface this by asking for forgiveness in advance, as I am new to solar, but not new to electronics. My take on the Growatt all-in-on system is that, from a philosophical standpoint, the main power source in the system is the battery bank. The reason I say this is because the Growatt won't even turn on without it, and it never seems to be idle. It is either being charged or discharging, and the job of the solar is to maintain the battery bank, thus the Growatt itself, and then power other loads with whatever is left over.
When the loads are small and/or inconsistent, the system starts having to make decisions on which parts of itself to use in order to minimize losses and keep efficiency at the highest levels possible. So it may be that, the given load on the system, the rate at which the batteries discharge, etc..., cause the Growatt to decide that the losses from powering up the voltage converter for solar when the batteries are at near or full charge, are an unnecessary expenditure. This may be especially true when there is abundant solar energy available, so it "knows" that whatever power it pulls from the batteries can be recovered quickly when the batteries reach a lower-than-full SOC. So it takes the route of highest efficiency, and turns off the solar converter, until it's use is necessary.
Now to the problem. The Growatt doesn't have a LifePo4 battery setting, so it isn't really aware of the battery bank characteristics. If your loads aren't high enough to tax the batteries, it may take it longer to decide that it's time bring in the solar. The solution in my case, was to simply increase the size and frequency of the load. The moment I connected a refrigerator (and now, a small freezer) this issue seemed to disappear. I suppose one could fiddle with the numbers to minimize this behavior, but doing so at the possible expense of total capacity might be somewhat counter-productive. Perhaps the addition of a useful, constant load, and maybe a timer so it only kicks on during daylight hours would be enough to sort it out.
My solution was to tax the system and let it do it's thing. But if the system never returns to solar and leaves you with half-discharged batteries at the end of the production bearing day, then there are likely other issues at play.