First, did you try clearing the browser cache? Trying in your browser’s Guest mode helps eliminate the suspicion that it could be a cache issue.
We need to debug the issue to understand what’s causing the Validation Rules not to work for certain users. Do you see any errors in the Console/Network tabs in the DevTools (F12)?
It’s important that we can reproduce this issue so it’d be helpful if you can share the configuration settings for the Validation rule. What type of metadata object is used in the Left and Right sides of the validation rule (and what are the sharing settings for these objects)? Are organisation unit levels selected?
Is this issue for all Datasets or certain datasets?
It would be great if we can reproduce this on play.dhis2.org.