Homekit Thermostat Only Behaving Badly when using SIRI or Home App

Every time I change the temp on my Ecobee Thermostat using Siri it does something weird.

Thermostats have 4 modes - Cool, Heat, Auto, or Off. Cool assumes your just using the AC, Heat the furnace, Auto adjusts to the set temp regardless of whether it needs AC or Furnace to achieve it, and Off is not going to manage temp at all.

When I set to "Cool" it should stay in the cool mode, regardless of what UI I'm using - Home App, Ecobee App, Siri (also Home), or the Thermostat directly.

Let's say we have the stat set to Cool mode, and at 74º. If I tell Siri "lower temperature 1º it will turn on the AC, but then we ultimately start getting too warm and discover it's now on Auto, not Cool, and the Temp range is too wide to be useful - like 78º or something weird like that. If we set the mode back to Cool, no matter how we do it - from Home, or the Thermostat directly, the very next time we tell Siri to raise or lower the temp, it is back to Auto, and useless ranges. We don't typically tell Siri to set to a specific temperature because if we have to do that we have to know what the current temp is. The only way to do that usually is by opening the Home app (or the Ecobee App), or looking at the widget, and then amending the verbal command to set to a specific temp. If we have to open the app anyway, what's the point of telling Siri? I mean - a lot of the times it's when we're watching an Apple TV, and we use the Siri Remote. The phone might not be nearby, so how do we know what the temperature in the room is? All we know is we're uncomfortable and want to raise or lower it.

Mainly, the behavior I want to stop is for Home to stop changing the mode from Cool to Auto (or maybe Heat to Auto in the Winter). This wasn't a problem until recently.