Mysterious Bloody Mary Deaths

[Saltinthedesert]

I’ve had a cherry shrimp tank going since Covid started. Added some Bloody Mary’s 6 weeks ago and they have done great. It’s fairly planted and I have chili rasboras and a pleco in there. 
 

I had the tank around 77° to encourage breeding but haven’t seen results so a few days ago I turned the heater down and it slowly dropped down to 75° over 2 days. 
 

Both of my highly colored females have died in the last 24 hours. I’m a little baffled. Has anyone heard of such a subtle temperature change being lethal?

[Saltinthedesert]

Also, no ammonia spikes, KH, GH, PH, and TDS have been consistent. 
 

Only the Bloody’s died. Other reds are fine.