We noticed that our CD environment gave different results from the index compared to the CM environment. We did some more checks and found that the index on CD was also not updating anymore. It did work before. We also noticed it's not all the indexes that have this issue, only our SXA web index.
We did have a deploy but nothing changed to the index configuration - which is rather default actually.
We had quite some issues with indexes before, but not this. We also can't reproduce it on other environments.
After rebuilding the affected index(es), the issue was fixed. But I would like to know what is happening as this might occur again.