A new feature has been implemented in postmarketOS edge (!2155): new U-Boot versions will be rolled out after upgrading u-boot-pinephone, which is used for both the PinePhone and PineTab.
Before this change, the bootloader would only get updated after running update-u-boot manually (which is still possible and can be used to change the RAM clock frequency) or when flashing a new postmarketOS image.
Great care has been taken to ensure that this works, there's even a checksum verification after rolling out the new version. If something should go wrong however, the wiki explains how to restore U-Boot.
This change has been implemented, so we can ensure everybody is on latest crust-firmware/arm-trusted-firmware before upgrading to the 5.12 series of megi's kernel tree. He noted that otherwise phones will overheat.