It sounds like you're experiencing sensor readout artifacts or banding, which can happen in low-light situations, especially when pushing exposure in post-processing.
Possible Causes:
Sensor Noise & Readout Issues
Electronic Interference
Long Exposure / High ISO
If this purple line consistently appears in dark areas or only after pushing exposure, it's likely a sensor characteristic rather than something you're doing wrong. If it worsens over time, it could indicate a developing sensor issue.