2007 Mazda 3 Replaced Thermostat Now Cool Air on Idle

2007 Mazda 3 - Threw a CEL - (P0128 - Coolant
Thermostat (Coolant Temp Below Thermostat Regulating
Temperature)

Fixed CEL by Replacing Thermostat -

I Flushed Coolant and Replaced Thermostat -


                   New Problem -
-After replacing thermostat and adding new coolant -
Heater blows cool air when at idle even when temp is
engine temp is normal. When car engine reaches 2,500+
RPM it blows hot air. As soon as I come to a stop it blows
cool air (slightly warm). (This was not an issue prior to
replacing thermostat & coolant)

                       Things I’ve attempted -
-Iv’e attempted to "belch" the coolant lines - I’m not sure
if I’ve done it properly but it appears to fixed it about
25%-50%

-Also checked to make sure that the coolant level was at
the correct level, while the engine was both cold and
warm.

Thing’s i’ve checked & I don’t know if it could be
indication of whats going on)
-The Coolant reservoir appears to be reaching
temperature and creating pressure. - I’ve unscrewed cap
after car has warmed up and pressure has released.

-At one point I had Coolant reservoir cap off while car was
idling and coolant overflowed (Not sure if it’s supposed to
do that)

-After car is warmed up - I noticed only one of the two
hoses coming from the thermostat feels warm to the
touch.

-Replacing the OEM thermostat, looked slightly different
from the new one I put on. The difference appeared to be
a hole on the side of it (Possibly for burping the coolant?)

-Iv’e seen no signs of leaks, or anything like that as of
right now.