? electrical problem...what’s happening? Should I worry?

Your converter does seem to be working more or less properly, at least at the moment. About the only other easy check would be to turn off the AC power and see how it reacts to loading only on battery power (no need to run the battery down too much--just see if the battery voltage and brightness both stay in the reasonable range as you turn on more lights, etc).

You may very well have had a loose/poor connection at the battery that happened to get corrected in the process of checking things out.