V15 detect absolute - strange battery behaviour.

Purchased April 2023. Recently my 2 batteries and then the motor unit were replaced by Dyson under warranty. Noticed a couple of days ago that the blue charging lights were still flashing after being on the dock about 14 hours. Turned the machine on, the remaining battery runtime was jumping about wildly, and within a couple of minutes the screen display showed low runtime. I put it back on the dock and the charge indicator  immediately showed 75% charged. Left it on charge until the blue charging lights stopped flashing. Switched on again. Runtime started at 70 minutes and within a second dropped to 3 minutes, then showed low runtime, and fluctuated between 5 minutes and 1 minute. Then the red battery symbol came on and stayed on whilst I continued using the machine for another 10 minutes whilst i finished vacuuming. I put it back on the dock, at which point the charging indicator showed the battery to be 45% charged. Swapped the batteries over. The second battery is doing the same wild runtime fluctuations 48, 22, 18, 36, 46 etc. I ran this until the battery went flat and it stopped, and the runtime seemed short to me. When I put it on to charge, it showed as empty. So is this likely to be a battery issue, or a machine issue? I use the machine on the auto setting primarily, and rarely use boost. I had no bother with the Dyson until fairly recently and I'm getting fed up now as I just want a vacuum thst does the job without me having to spend ages on the phone with support or writing about it.