I found this post and there are several possible solutions mentioned in the comments. The easiest one to try being:
I was having a similar issue. What worked for me was going into the web map details, un-checking "Enable Offline Mode," saving, edit again, re-check "Enable Offline Mode" and re-share with the group the map was participating in.
Maybe give that a try and see if it helps....also, make sure there are no copies of layers or layers originating from the same data source in your map.
Best,
Katie
“The goal is not simply to ‘work hard, play hard.’ The goal is to make our work and our play indistinguishable.”
- Simon Sinek