Scanning Mailing List I think it has to do with
New XML format for project info files
It appears as thiough the new format is auto applied when opening an existing project, which kills the build modes defined. The changes are made even if you do not save the project.
If this is the case then any project, demo that I work on using Trunk will break the project for any one else that is not using trunk.
In my view the New format should only be applied if you explicitly tell it you want the new format, else it should be the previous version.
Ideally when such a change is going to be made, a confirmation dialog should be displayed.
Do you want to Upgrade your Project to New Format?
Or Keep it in Compatibility Mode?
Then store this setting somehwere, so your not continually prompted when opening that project.
Also have option in Project Options to convert to New format; when all your lazarus builds are capable of using the new format.
I managed to open a backup of my effected projects, and export the builds modes etc xml, and then open in Older Lazarus and re-import the build modes etc so that I can use the project.
Until the change is corrected, I cannot see me doing further testing on trunk, tooo destructive.