<Puts on RF designer hat>
Although the 5G transmitters are within their specification, RF radiation does not suddenly stop at an arbitrary frequency.
There will still be some 5G transmit energy in the radio altimeter band, and although that level is fairly low, most radio altimeters (regardless of the method used, either pulsed or frequency swept which are the primary techniques) have a receiver sensitivity of around -100dBm.
Put that in perspective; that is 100 femtowatts which is not enough to warm a gnat's behind.
Adding filters won't really help the affected devices very much. All that will do is reduce the resolution of the device and although that may be ok,, it is a material change of specification of a safety critical item which requires re-qualification to level A (failure can result in catastrophic loss of life).
So whether a new piece of equipment or an update to existing equipment is done, there is an expensive piece of work to be done.
The 5G antenna could be adjusted to not point at the sky, of course, which would also solve the problem.