It seems extremely unlikely that the technology was gradually and seperately lost on both sides.
Maybe, as the desert expanded, the seafaring people moved north and south to more hospitable places. Leaving nobody in West Sahara to recontact the Canaries.
Are you mistaking them with the Azores?
Politically its part of Spain of course.
Ancient Mediterranean was full of naval cultures. But when the Roman Empire declined and fell in the west, maritime trade declined as well. Then Arabs conquered North Africa, turning the Mediterranean into a battleground. Nobody cared anymore about some remote sparsely populated islands.
https://www.ing.iac.es//~crb/trav/lp/islandvis.html .. maaaaybe Fuerteventura.
"The African coast lies 124 km from the highest point on Fuerteventura, Pico de La Zarza, and 97 km from the closest point on Fuerteventura, Punta La Entallada. Assuming a height of ~ 100 m for land near the Moroccan coast (there are no mountains close to the coast), the predicted visibility ranges are 137 km from Pico de La Zarza and ~ 90 km from Punta La Entallada (and ~ 100 km from a peak just inland of Punta La Entallada). Africa should therefore be visible from at least Pico de La Zarza, on a clear day. "
In this case I'd say it's the exact opposite: on the islands, neighboring communities right on the horizon provided enormous pressure to maintain the ability. Continentals on the other hand have very little incentive. I'd rather say by the time islanders have lost the ability, continentals will most likely have lost it ten times over.
Maybe motivation for such a risky sea crossing was just gone. Besides there was probably a good chance one would get hit on the head and have their ship and produce/merchandise stolen by whistling Canary headbangers. All for what? A handful of barley and some odd purple dye?
FTFY.