I don't think they can immediately apply the regen because every other ground-targeted field is reliant on the server tic as well. Seems like it's a hard coded thing.

One solution is the allow the AST to throw up Collective Unconscious but if the AST is interrupted or they end the bubble prematurely, the bubble will persist for 3 seconds to ensure the bubble dishes out its tic. You'll be able to cast magic while the bubble persists for the previously mentioned duration.