Thanks for the replies.
Her irregular pulse has been with her for most of her life. As mentioned in the first post, it can be very severe but has never affected her (high level of) fitness.
This missing pulse is new.
The loss of pulse in her left arm is very, very near to being none. Tiny traces can be felt, but the machine(s) just read nonsense.
Blood has to be getting there because of the strength of her arm and the grip of her hand. If one wasn't looking for a pulse, one would never know anything was wrong.
One is mystified.