If too many tasks is given to QtConcurrent, it does nothing. It should at least report that something went wrong.
The issue can be reproduced for example with Qt Online installer version 3.0.4 or lower, which gives over 3000 download tasks to QtConcurrent. The issue can be reproduced only in some virtual machines or in machines which low performance. For testing purposes I created a custom Qt Online installer which gives only half of the download tasks to QtConcurrent and then the issue does not occur.