Hello, as a AMS Pro and W3 total cache user i haved the same issue, solved! but…
After adding the user agent list rejection the switching works again, Good! But unfortnnaly i notice that caching woks now only for destktop version and not mobile. is there a possibility to have cache also work for mobile?
In case if it could help:
When the issue was present, i notice that the same theme was effective only in front page (desktop and mobile). for other page rooting between themes with catching worked well.