It is also said by Kylo that the part of the map in R2’s memory was from the records of the Empire, presumably downloaded while on board the Death Star. This would mean that either the piece held by Sydow was charted later, or the yellow line marking the path to the temple was the only information yet to be discovered at that time. Perhaps Luke copied that small section of map from R2 then deleted it so that nobody in the Resistance could even start looking for him, then when he discovered the route to the temple, he gave the route to Sydow.