There is nothing wrong with equipment and it definitely is NOT alien interference.
Most Radio Altimeters are design limited for height range. Most will flag and remove the height indication when beyond design limits, but not all. Some use different FMCW processing techniques to provide limited “look ahead” features.
Some Radio Altimeters designed for special applications such as guided weapons and military aircraft use PCM instead of FMCW. These can also have different range design features. These are by definition much more expensive and PCM altimeters are not common in civil aircraft although I have seen them installed for special operational applications.
Many are unreliable overwater particularly if the surface is flat calm, the same can happen over reflective sand based terrain. Antenna position and in particular antenna spacing and surface induced discontinuities are optimised for individual aircraft and are or should be in compliance with the equipment manufacturer installation specifications. There should be FM Supplements and or operating instructions for individual aircraft.
Sincerely, Avionics Design and Systems Integration Engineer.