I haven’t done lots of testing by any means, but I also didn’t have any problems loading lots of thumbnails in my testing.
I think it might perhaps have to do with the fact you’re using pcall, which is usually a good idea but the getthumbnail function actually does wonders by checking if isReady is true instead of success from pcall. So I’d recommend testing if it works by doing it the same way as the docs have done it since they don’t use pcall there