The FFT indicates this is a periodic signal, not a once a day or week thing. It's happening continuously all day every day. I don't know of any UPS systems that perform tests with this kind of regularity.
In any case, I refuse to believe an external sync signal is causing this. The more likely explanation is that the active elements are pulling each other to a common mode due to being physically coupled on the same grid. This is like how clocks on the same beam will synchronize.
From the article:
> The origin of these current peaks was independently confirmed by the manufacturer to be periodic self-tests executed by the battery EMS, confirming our interpretation. Similar minute-scale diagnostic routines are also documented for other commercial UPS systems (cf. Supplementary Note 8).
https://www.snaptec.com.au/akkutec-4801-din-user-manual.pdf
> battery circuit testing 1 x per minute for 1 sec
https://adelsystem.com/hubfs/PDF%20DC-UPS/Instruction%20Manu...
> The battery testing is carried out automatically every 60 sec.
Their testing on the enterprise-class equipment shows the difference that NTP makes, and with enterprise-class equipment comes wider deployment and more powerful models.
But most of the consumer units would not have this stuff. Surely, the manuals would describe how to set it up.