The capacitor is assumed to have no charge at time=0.
The capacitor charges from the output. The only way charging can take place, is if it can pass current in one direction or the other. The op amp is labelled 'ideal' therefore its input has infinite resistance.
Therefore charging current needs to be able to travel from or into Vin. However the schematic does not specifically indicate this is possible.
Therefore we must assume Vin is a voltage source whose unseen terminal is connected to 0V ground. And R1 is the total resistance in that path.
what CAD are you using?? Are you shure of the connections? IMHO there are some problems in the schematics and note that you are using uV as input signal, i think that they are too small!!! try at least some x10mV
Why from -25 to -28 V on the output graph has shape like a decreasing exponential ? It is not supposed to be a line with slope -1/CR until reaches lower supply rail ≈-28 V ?