Deleting recordings does not free up USB memory.

I have a Philips Oled806 with Android TV. I use a USB stick in it to record via the satellite tuner. This has always worked well. Recently, however, the memory has started to fill up. Deleting recordings does not really free up memory. In the meantime, there is only one recording left and only 6 per cent is free. It's a 32 GB stick. When I formatted it for use, about 80 per cent was free.

Does anyone know the problem and perhaps a solution? I don't want to reformat the stick because I want to keep the recording.