Steps to reproduce the issue

1. on a site with J5.4.0-beta1 select update channel 'next' and set stability to release candidate
2. save & close: you are prompted that there is an update for 6.0.0-beta1 yeay!
3. perform the update

Expected result

updated Joomla 6.0.0-beta1 site

Actual result

downgraded (and crashing) Joomla 5.2.1 site

System information (as much as possible)

Additional comments

from the update log file...

2025-08-21T11:15:05+00:00	INFO --.--.--.--	update	Test logging
2025-08-21T11:15:05+00:00	INFO --.--.--.--	update	Update started by user Super User (934). Old version is 5.4.0-beta1.
2025-08-21T11:15:06+00:00	INFO --.--.--.--	update	Attempting to download update file from https://update.joomla.org/releases/5.1.2/Joomla_5.1.2-Stable-Update_Package.zip.
2025-08-21T11:15:07+00:00	INFO --.--.--.--	update	File Joomla_5.1.2-Stable-Update_Package.zip downloaded.
2025-08-21T11:15:07+00:00	INFO --.--.--.--	update	Starting installation of new version.