If it is regarding to the app build in the cloud, see the answer in this post:
I'm having this same issue with our very own Mendix Forum as well.
It seems to be that modern browsers are caching the index.html way too aggressively and with that the cachebust. So the cachebust doesn't really get a chance to be updated, resulting in an older client being loaded from the cache.
We're trying to find a solution for this. In the mean time I suggest you create a ticket so Support knows who this issue is affecting and you will be kept up to date on it.
New improvement of Mx 7.4 (see release notes):
See answer Robert ^
We observed same issue after upgrading to 7.4. It happens to us on Firefox browser. IE it works fine.