Ocean monuments that have been cleared and replaced in the spawning range with guardian farm designs will not spawn guardians upon transferring. This issue is also persistent with ocean temples that have been left alone upon transferring from Minecraft Xbox One Edition to Minecraft Bedrock for Xbox One. 

I just imported an old world that I've been playing on for 9+ months, where, prior to transferring and playing it on the older Xbox One Edition, the farm had moderate rates. Upon transferring, no guardians spawn once so ever. I also checked and tested other local monuments that were inside of the initial world boundaries, and that still holds true. With temples generated from the Bedrock engine (outside of the initial boarders), there were guardians spawning, but the rates were very poor (and this did not have anything to do with unlit caves or drowned spawns). 

I've been holding back from playing on Minecraft Bedrock due to the plethora of bugs and broken mechanics (especially redstone), and this is one other reason why I'm still incredibly apprehensive from playing on it. Please look in to this, it sucks to not have a highly reliable source of EXP and prismarine, and have all of my hard work clearing the ocean monument wasted.