Underlying Technical Details
It's important to understand that ARNS (Aeronautical Radio-Navigation Service) operates in the 4200 - 4400 MHz range for transmit and receive. The issue appears to be a combination of ARNS receivers being sensitive to (strong) signals outside the immediate band and the cellular base-station signal strength in 3700 - 4000 MHz.
Historically the band has been used for low-power services that do not suffer 'bleed' so ARNS receiver design especially didn't require tight band-pass filtering.
Frequency Allocations:  slide 7
3700-4000MHz Fixed Mobile
4000-4200MHz Fixed Satellite
4200-4400MHz Aeronautical Radio-Navigation
"It should be understood then that any interference that is unpredictable and that can mix with the linear FM waveform, thereby causing the radio altimeter to mistake the mixed signal as terrain has the potential to cause a radio altimeter to report a false altitude. "
 page 9 "1.1 Radio altimeter modulation and receiver sensitivity"
"All FAA Part 135 helicopters are now required to have an operational radio altimeter
◦ Approx. 22,000 operational civil rotorcraft
◦ Some FAA Part 91 aircraft require altimeters for certain operations such as Cat II ILS, etc.
◦ Approx. 34,000 general aviation/private aircraft
◦ All large passenger aircraft
◦ Approx. 7000 US based civil aircraft
◦ Plus international carrier"
 slide 5 "Equipage and operation US National Example"
 ITU-R M.2059-0 "Operational and technical characteristics and protection criteria of radio
altimeters utilizing the band 4 200-4 400 MHz" https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.2059-0-201402-I!!PDF-E.pdf
 "Radio Altimeter Interference" https://www.icao.int/NACC/Documents/Meetings/2018/RPG/RPGITUWRC2019-P08.pdf
 "FAA Radar Altimeter and Compatibility with 5G presentation" https://rotor.org/wp-content/uploads/2021/08/FAA-Presentation-RA-5G-Industry-Forum-July-2021.pdf