I do not understand why someone would change this without providing an alternative.
You're using BUILD_ID for something other than identifying builds, which it is meant for. So sure, it was documented as being a time stamp, but it's not unencumbered with other uses.
Breaking someone's code with no compelling reason seems, well just wrong.
The previous BUILD_ID format required tons of additional code and had several limitations, which have all been fixed by switching to build numbers. Looks like a compelling reason to me.
Best would be to change it back to the way it was. OK would be to provide a new environment variable. Preferably with the same format.
Neither going back to the old storage format, nor using a value for BUILD_ID which isn't actually the build ID, are sane options going forward IMO. A new variable should be easy to add, if maybe only in a plugin.