There are several complications; see here: https://lwn.net/Articles/984249/.

For example, if the copyright holder applies a security fix to an old version that had "expired" and is now open source, will that cause old version to revert to source-available? Does any security fix, even the simplest one, require clean room reverse engineering on part of the community? Unless these questions are answered clearly by the copyright holder, BUSL/FSL are not really usable as open source even after the expiration date.