I have read a lot of IAEA documents. I can't recall seeing anything in them that places requirements on an invading army to report unusual conditions. Nearly all of the requirements are on the country or agency operating the reactor.
Countries that operate nuclear reactors are generally required to report anything that could affect the safe operation of their own reactors. It becomes automatic. Somebody makes a mistake in maintenance procedures that is caught by the follow-up check - report to the IAEA. The country's currency fluctuates meaning purchasing nuclear fuel may be difficult - report to the IAEA. It's too dry so the river/lake used for cooling is low - report to the IAEA. It's too wet so the lake is too high possibly making it difficult for workers to safely get to the station - report to the IAEA. The wrong surface-treatment cement sealant is delivered to the station but not used - report to the IAEA. There is labor unrest in the catering industry in the nearby town so workers may not get their cafeteria meals - report to the IAEA.
It becomes a habit.
I'm thinking that Russia is just following routine. They see a nuclear reactor. Something is happening near it. They report it to the IAEA.
Not that the IAEA is likely to be able to do anything about it. In most cases, countries have their own national standards and enforcement agencies. In Ukraine it is the State Nuclear Regulatory Inspectorate of Ukraine. This agency is, understandably, hampered by the invasion.