Hello there @tomwilson88
We didn’t have any previous report like this, it seems like a conflict with another plugin or theme.
Could you please proceed with a conflict test? First create a recent backup in case you lose any settings during the test and have to restore to a previous working state.
Then deactivate all plugins and MU-plugins apart from Smush, activate a default theme like TwentySeventeen, clear all caches and check again if the issue is still there. If not, it means that something is conflicting, so start activating the remaining plugins and theme, one by one this time, checking every time for the reported issue, until you reproduce the issue, thus finding the conflicted combination. The following flow chart image can assist you during this test: https://premium.wpmudev.org/wp-content/uploads/2015/09/Support-Process-Support-Process.gif
If this is a live website with traffic, I’d rather advice to create a staging environment, meaning a complete and exact copy of this installation into another location/folder in the same server, using a separate database, and perform the test in there instead without worrying about live site.
Thank you,
Dimitris
