I live in a condo, 1 bedroom 1 bath which has its own direct vent 50 gallon water heater. The condo also has a heat pump which utilizes water from the Hot water heater.

When running the shower for more than 15 min, the hot water will suddenly go cold. If I try the sink in the bathroom, it has hot water but the shower does not. I replaced the cartridge within the Delta Converter in the shower thinking that the converter was causing the issue but unfortunately the issue still occurs. Any ideas what would make just the shower suddenly run cold after an extended running of hot water. I know that the tank can't be out of hot water because the sink within the bathroom still has hot water.

I have no idea why it would take 15 minutes, but I'd try isolating the shower from other fixtures and systems, one at a time. Close the stop valves under the sink in the bathroom...test the shower. Valve off the heat pump....test the shower.

I would replace the check valve at the water heater / heat pump
can you upload a few photos of that hybrid system?

Try replacing the valve or you can call a plumber to fix it.