@Jonathan Sharp Welcome to Microsoft Q&A, Thank you for posting your here!
It could be portal issue, I was able to repro the issue in my lab and able change the replication options under configuration, can you please try to clear cache, temp files and also logout from the portal(You can also try with different browsers) and try again, If the issue still persist, can you please share the screenshot.
Additional information: For testing purpose your can use Azure PowerShell to perform this action.
Configure object replication for block blobs
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
--------------------------------------------------------------------------------------------------------------------------------------
Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.