I was on my skyblock and I asked Delvin to help me with my /is party problem. And he found that I had people from previous islands still added I thought maybe I should kick them. And when I kicked all of them that weren't on my island any more my glitch was gone. I think that could be a way to fix some peoples island party glitches.
Some people's islands glitch where it doesn't show them their island party, so they may not always work
That was also what was happening I asked delvin if he saw anything different and he did. I originally only had 5 on my is it said I had 9 when he looked. But when I did the command I only saw me and 1 other