This has happened to me too. That’s why I now switch out the Bender detector with one I know is working after the most probable causes have been investigated.
Last year during the coronavirus mayhem I took a temp job on a vesse that had a bad Bender detector (10 yr old) on a 480v switchboard. I reported the issue in the companies maintenance program. I didn’t have to switch it out with another one to test because it showed a ground with nothing connected to it. It was decided to send it ashore to attempt to be repaired. The problem I had was a lengthy ABS inspection was to take place & I didn’t want to have an empty 4" square hole in a switchboard during the inspection so placed one of those glow in the dark, high voltage warning IMO stickers on top of it. It looked almost completely normal. Even during all the standard reverse power, under current etc. tests the ABS inspector never said a word. If he had, I would have told him about it being written up waiting for parts. It couldn’t be repaired, a new one ordered & when I left the sticker was still covering the hole.