Ya, momentarily it'll be bridged together when 2 rollers touch different blocks as it's arriving. We've found that it's okay for any engine to briefly move across the different blocks that are using different voltages. In our case we have a ZWL for the 2 main loops, and we use a separate transformer for the accessory power for the switch machines and relays, but they all share the same common wire.
Since the start/stop block is always off with the way the time delay relay and z-stuff relay is setup, the engine will fully stop when all the rollers of the engine are in the start/stop block. In reality we created the start/stop block to be 30 inches in length to comfortably hold a large engine including the Big Boy and to account for rolling stops from conventional engines that do not have cruise control technology such as Odyssey.
Another method we used to combat the bridging of the two blocks together for a period of time is to put a car behind the engine that do not have rollers on it. For our passenger car trains they have either a baggage car or box car that'll trail the engine. The reason for all of this precaution and is the ONLY situation bridging does something odd. When the time delay relay board is powered off and the train is arriving to stop the engine bridges the two different voltage blocks together (stop(0v) and 10v) and the time delay relay board freaks out because it is receiving power through the relay in the reverse direction it's built for. When the voltage bridge is happening anywhere else on the layout or the time delay relay board is on, like when the engine+passenger cars depart there is never an issue of the time delay relay board freaking out.
We've been running this for almost 3 years like that and haven't seen anything break or have ill effects. But your question actually made me think of an idea that didn't cross my mind. I should add a diode that can only allow power one direction to the start/stop block. Thanks for that.
Also I can search for a video example if you're interested and I hope my explanation answered your question. This whole thing has been a learning opportunity for me since 2015.
