Text styles not applied after Load Style...
There are several bug reports similar to this issue (particularly #24436: Changes made in Text Properties do not affect all selected elements). It may be that the problems that occur after a Load Style... are related, but it is not clear as 1) there are no text elements selected when loading a style and 2) if the text elements are selected, a Reset Text to Style seems to fix the problem for all selected elements (which doesn't sound like the problem in #24436: Changes made in Text Properties do not affect all selected elements).
I also think I saw some comments that said that this problem occurred when a MuseScore 1 file was loaded into MuseScore 2.
In my case, I created a file using MuseScore 2. This is the first attachment. I then modified the styles. The resulting file is the second attachment. I saved the styles and this is the third attachment. Finally, I reloaded the original file (before its styles were changed) and executed Load Style... The result is the fourth attachment: nothing changed. Selecting each element type and clicking on Reset Text to Style applies the new styles to the existing elements. This is a pain as there are a lot of text styles.
The general styles appear to be applied immediately.
If this was working correctly, the second and fourth attachments would look the same. In the fourth attachment, new elements do get the loaded style.
Apologies if there is actually an existing issue for this. I tried a variety of search terms and even looked in all issues, not just the open ones. I never found an issue that I thought was exactly the same as what I am reporting, but I may have missed it.