This issue has been in OpenEMR for quite some time (I remember it in the 5.0 release). It is still there, and I have worked around it by going to faster instances that have a faster patient chart switch time (like t2.large for example - hopefully, t3.large will be enabled soon).
The problem is not one that developers would typically see on a test. It requires a system that is loaded with normal use (several users).
The issue is that some of the information (like the buttons for the encounter, etc.) will stay “connected” to the previously viewed patient chart when the patient is changed. The webserver will update the current chart and then the encounter, demographics, and other connections will update.
Between the time that the patient chart is changed to a new user and these other connected pieces of the chart switch to the new patient, if the user tries to edit something, they will be working in a different patient than what OpenEMR screen is showing them (the patient name and the patient that is pulled up when the user chooses demographics etc. will not be the same). I remember getting burned by this many times when I was using slower web server instances.
The problem gets even worse if you try to have multiple patients open with the same browser (stick to one patient per browser - open a different web browser to view a different patient) - with multiple patient charts on the same browser, the OpenEMR system can get into a state where the demographics, encounters, etc. are attached to a previous patient and never update at all (requires logout and login).