Similar to Why can't the Jedi detect Palpatine's Force sensitivity? but focused on the Legends version only.
Plagueis discovered Palpatine only at the age of seventeen. It took some time before Plagueis discovered Palpatine's strength in the Force because of the barriers the latter had built up over the years to hide his teen angst.
However, the Jedi are supposed to be able to know every single Force sensitive in existence within the Republic through midichlorian tests that are instituted within all routine medical tests at birth. Naboo is a Republic member world from the Mid Rim. Palpatine's father was not a Force user, and had no contact with the Sith Lords at the point of Palpatine's birth.
How did Palpatine's undeniably high midichlorian count evade Jedi attention when he should have been subjected to a Republic medical checkup at birth, and there were no Sith present to mask the results?