An early solution freeze leads to a superior proposal, with an adequate solution.
A late solution freeze stresses the proposal team and often results in a poorly written proposal fraught with pricing inconsistencies. No one can clearly describe something that keeps changing. Inter-volume alignment is lost and contributors are repeatedly required to re-work their drafts, time estimates, and materials estimates.
Worse, contributors adapt by having to describe the solution in general, vague terms. As a result, evaluators reduce your evaluation score, and your contractual risk increases.
Determine who has the authority to freeze the solution, set the date, and make it happen.