On Monday, we reported on Apple's point update release of iOS 12.5.1 for older devices which cannot run iOS 13 and higher. The only stated change in the update was a fix for a bug in the COVID-19 exposure notifications feature, which Apple had previously added to older firmwares to allow more device owners to make use of it.
Although the release notes didn't mention any other changes, it would not have been unusual for Apple to have taken the opportunity to patch the vulnerabilities used by iOS 12.x jailbreaks, such as the Chimera jailbreak for iOS 12-12.5. The update could also have rendered the jailbreak tool unusable accidentally. Unrelated changes in the firmware, or hard-coded firmware support in the Chimera app itself, could also have required an update to the tool for it to work again.