You can do the requested task using not Select by location but Select by expression.
In the image below you can see one example of the necessary expression that selects all the features that have an X coordinate smaller than the X_min (minimum X coordinate) of the polygon.
This selects all the features on the left side.
This is the expression:
I solved the problem, though I'm still unsure what caused it.
I was testing to see if there were any projection issues, and found that there were with the following code:
print(lochit.crs().isValid()) # True
print(lochit.crs().authid()) # -blank-
For every date beyond the first in the loop, the crs is found to be valid, but authid() has no output. ...