Typically that's caused by corrosion between an antenna and the structure. It needs a really good radio frequency bond there. The fix is to drop the antennas, inspect and replace any corrosion damaged antennas.
Less common is a leaky connection between the unit and an antenna cable connector in the mating connector.
RF leakage is noticed first at altitude, because the transceiver is operating at max gain, and any leakage will show up there first. Once there is ground return greater than the leakage signal, the gain is reduced and the altitude is accurate. Hence, it's not by itself a safety issue.
Depending on the leakage, you may see a flag, or as in the case of AMS, a negative altitude with no flag in view.
Aircraft systems are built to accommodate failures such as yours, but less likely to accommodate erroneous inputs with no warning flag.
In both your case and THY, additional line maintenance technician training and Maintenance Control training is sorely needed. Heavy check procedures may have been skipped, too.
GB