Looking at the code, I can see no changes between mIRC v7.51 and v7.52 in how the treebar is handled. There have been no changes to how clicking on a channel in the treebar works.

In situations like this, the only way to track down the issue is for you to test mIRC using a minimal installation, with no scripts/DLLs/addons, to see if the issue still happens. If it does not, the issue is likely due to a script/DLL/addon.

Also, mIRC has no feature where clicking on an item in the treebar minimizes the whole of mIRC, which seems to be what is happening in your video, so it may be that another application is interferring in some way.