Reproduction steps:
1. Open the chromium project: https://chromium.googlesource.com/chromium/src/+/lkcr/docs/qtcreator.md

2. Let it finish "Parsing C/C++ Files".

Notice: In htop or a similar utility, see system RAM usage increased by 4 GB since Qt Creator started.

3. Navigate around in the code a little (jump between FocusController.cpp, Document.cpp and 3-5 other files).

Notice: RAM increased with ~ 1 GB. That feels like a lot?

4. Close all editors.

RAM decreased with almost   ~ 1 GB.

5. File -> Close All Projects And Editors

Notice: htop shows no RAM drop. Looks like we're leaking here.

6. Open the chromium project again.

7. Let it finish "Parsing C/C++ Files".

Notice: RAM usage increases further.