Good evening Nicolas and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about this behaviour on your installation.
I would love to help you with this topic.
On my end, I am not able to reproduce the issue. When I go to a previous event (e.g. April) and hit the browser back button, I am brought back to April (since the URL call for it – https://www.waltonoptions.org/events/2017-04/).
Would you mind telling which URL gets displayed on your end when you press the browser back button ? And also tell me which browser on which OS you are using ?
Hang in there!
Geoff B.