I have a 2010 Kubota RTV 900 that won’t hold a charge. The battery was replaced about 1 year ago, it worked fine for a while, but now it has to be bypassed to start. The battery is now reading 12.9V after jumping and running, without the engine running and 14V when running. It will start up after leaving it running for about 30 minutes.
You seem to have a bad battery. It is charging with the proper voltage while it is running, but the battery is not taking a charge well. The mower battery is only guaranteed for 6 months, as they do not last like car batteries, nor do they charge as well.