You are not the only one :-) I am seeing this, too. It does not seem to have any harmful consequences, but the log spam leads to frame rate stutter when run through Xcode.

To further illustrate the issue, here is an annotated screenshot of the memory graph:

I can confirm that the issue has been fixed in macOS 15.2. Sadly, the problem was reintroduced in macOS 15.3, macOS 15.4 and 15.4.1. We could reproduce the problem on multiple Macs running 15.3, 15.4 and 15.4.1 – but it is working correctly on macOS 15.2. Is there any workaround? We are on the verge of conducting a beta test on TestFlight and testing in-app-purchases is crucial.

I can confirm that the issue has been fixed in macOS 15.2. Sadly, the problem was reintroduced in macOS 15.3, macOS 15.4 and 15.4.1. We could reproduce the problem on multiple Macs running 15.3, 15.4 and 15.4.1 – but it is working correctly on macOS 15.2. Is there any workaround? We are on the verge of conducting a beta test on TestFlight and testing in-app-purchases is crucial.

Strangely, I am not getting any customised thumbnail for a folder with an emoji using QLThumbnailGenerator. I am using the exact same code as the thread starter on macOS 26.1.