Importing CSV in QGIS results in inverted Y coordinates

Question

I recently did a surveying job using 2 Emlid Reach M2 receivers (a base and a rover). All points were taken using the NAD83 MTM 7 projection. When I try to import a CSV file in QGIS, the Y coordinates get inverted (48 degrees N becomes 48 degrees S). The points seem fine when I try to import them using the WGS84 projection, but it feels like they aren't as precise as they should be (data was post-processed and all points are supposed to be <1cm precise). What I don't understand is that the original projection in which the data is in results in wrong coordinates.

Is there any workaround to this, or is this the most precise my data will ever be?

Here is what I tried so far:

1. Import the corrected CSV into QGIS directly using MTM7 projection (inverts Y)
2. Import the corrected CSV into QGIS directly using WGS84 projection (points are not as precise as they should be)
3. Convert the corrected CSV into a SHP using a web app (results are the same as importing the CSV directly)

I would share the CSV file but it is sensitive data.