The faces where the armature and magnet meet are especially vulnerable to surface imperfections or oxidization that can greatly weaken the bond strength, especially one mounted outside.
See this video from Diagnose This Gate Access Problem:
Explanation from that discussion:
The video obviously shows the magnet and armature mating surface covered in corrosion. This is surprising to some, and the assumption is the maglock is composed of aluminum or stainless steel (not ferrous), yet rust on outdoor maglocks is fairly common.
Many maglock manufacturers warn that rusted bonding surfaces will weaken the lock strength, as noted in this Securitron FAQ:
In summary, the rust that forms on the mating surfaces weakens the pure electromagnetic bond that is possible, even if the internal components are unmarred.
The lock in the video may not be a Magnalock, but the plated construction and impact work-hardened rusting of maglock components is a general issue across many brands/models.
Like the note mentions, if maglocks are used outside, a periodic maintenance wipedown with a light oil is the only way to prevent inevitable rusting. It does not mean maglocks cannot be used outdoors, but it does mean that maglocks are not 'set and forget' devices that require no upkeep.
Interestingly, the member comments/ guesses about alignment being a potential issue are technically sound and could be correct. However, the alignment appears pretty solid and does not scream 'big problem!' like the appearance of rust.