Hello.
I have Voltronic InfiniSolar v4 (3.6KW) inverter, which is 48v inverter. 4x AGM batteries are attached to it (65Ah each).
Inverter is set to SUB mode (Solar -> Utility -> Batteries), so it shouldn't drain the batteries unless both Solar and Utility is unavailable. But what I see is:

That's the battery capacity data from the inverter, for two days. But I get almost the same picture for each sunny day - after 6am pv starts to deliver enough energy for charging the batteries up to 100%, then the capacity starts to go down very quickly, down to 60% before the next sunrise. And so on.

For the same period, the battery discharge current graph and grid voltage graph are:

I.e., during these periods when the capacity drops down, the grid is always available, and the battery discharge current looks like it's almost always zero.

But yet sometimes there are strange spikes like the one on the graph. Plus the inverter reports discharge current as integer in Amps, therefore I suspect if it was able to report it as a float, it won't be zero, looks like the real discharge current is not zero, it's somewhere between 0A and 1A.

I expected that in SUB mode the only thing that should drain the batteries is their self-discharge, which should be small enough for AGM, enough to have decent capacity even after a month without charging them. But in reality, after a couple of cloudy days I have the following picture:

Yeah, eventually down to zero. Very disappointing.

Does anyone know whether it is "normal" for inverters? Or am I having an issue with my hardware/batteries (what to check is that case)?

Well, any inverter has a "no-load" consumption. Typically around 50w, some even as high as 100w.

Also, as you are using AGMs (as opposed to Lithium), they have a self discharge.

You are correct that in SUB mode, loads will only be powered by battery if Solar or Utility is unavailable. But, the self consumption of these inverters seems to always be taken from the batteries.

I see the same behavior on MUST 3K 24V AIO inverter, even in UTI mode, which is basically like a UPS.

What you need to do is to enable Utility charging in addition to solar charging.

You can set it as low as 1 or 2 amps of Utility charging, so that self consumption will be taken from the grid and not the batteries.