You had a hard stretch at 9 months old, and then things improved. Now, at 18 months old, the pulling and the selective hearing are back.
It feels like being cheated. Nothing you built has been erased, and this is not the whole stage starting again.
What the research actually says
Dogs show reduced responsiveness to their caregiver's cues around the onset of adolescence, and that responsiveness returns later in the stage.
That work is about the early part of the stage. It does not describe a second dip at 18 months old, and we will not stretch it to say so.
Our reading of the later wobble
Our reading is that a lot of owners report a second rough patch between roughly 15 and 20 months old. That is a pattern we see, not a research finding.
Our reading is also that the causes are usually ordinary. Practice quietly stopped when things got easier, the long line came off, and the world got harder again.
The things that usually explain it
Check what you dropped. Most owners stop carrying food and stop paying the recall once the dog seems reliable.
Check the environment. An 18 month old dog goes further and faster, and meets more dogs, than a 9 month old did.
Check the routine. A house move, a new work schedule or less sniffing time shows up in behaviour within a fortnight.
Check for pain. Pain frequently presents as a change in behaviour rather than as obvious lameness.
Tonight, 10 minutes
Write down the 2 behaviours that got worse. Ignore everything else for 2 weeks.
Put food back in your pocket tomorrow, and pay the recall and the loose leash again for 14 days.
Put the long line back on for a fortnight, anywhere your dog has stopped answering.
Run one 5 minute session in the house tonight on a cue they are ignoring outdoors.
Going back to the version that worked is not a step backwards. It is usually the shortest route through a dip.
How long this part lasts
We will not put a date on it, because your dog is not an average. Adolescence runs roughly 6 to 24 months, and most owners describe the late part as steadier.
Safety note
A developmental dip is gradual and behavioural. Sudden change over days is different, and so is new fear, new growling or new biting. Book a veterinary examination first, then speak to a qualified behaviour professional. Pain often presents as behaviour change rather than limping.
Today's version, in one line: food back in your pocket, long line back on, and 2 behaviours for 2 weeks.