I understand the Player’s Handbook’s ruling on teleportation and attack of opportunity when teleporting out of someone’s reach. My question however is, does teleporting into someone’s reach when they have the Polarm Mastery feat provoke an attack of opportunity?
By RAW you do not provoke an Opportunity Attack when you enter their reach by teleportation.
You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. (PHB p.195)
It doesn't stipulate entering or leaving a creature's reach by teleportation. You do not provoke an OA when you teleport.
Polearm Master does not mention anything that is more specific that would override that rule.
RAW as per PHB195 specify the teleport does not trigger an AoO. However, after the teleport has concluded if you appear in the targets threat range, that is a trigger for the Polearm Master as you are entering their reach.
Based on this information I would suggest that teleporting out of or within the threat range would not trigged an AoO but teleporting in would.