Junk mail keeps coming back from Junk into Inbox

For about a month now, every so often, things that were moved to junk (sometimes by me, sometimes because I have previously marked them as junk) will reappear in my inbox. Usually, it is around 40-50 messages at a time and covers a period of the last few days.

Today it did it, but there were still new messages in junk, so it looks like that bit works ok, randomly though things just then come back into my inbox.

I can keep deleting junk so there is nothing to reappear, but does anyone know if this is a known issue that will resolve itself or something I need to fix? Thanks.