I've got quite a fair share of warped miniatures and barricades, so I did as described in LTP and submerged the items in hot water ( 55-60 C) for 10-15 secs and then corrected the shape and immediately transferred it to a bowl of cold water.

At first I thought that it was pleasantly easy to correct any bend or warped pieces but after a couple of hours I noticed that all the pieces had warped back again.

Did any of you experience something similar or am I doing something wrong.

Not had your problem with Legion but in the past I have used a hairdryer to heat up plastic and then bend to correct shape.

Yeah if they delay warp back, you need more heat for longer and perhaps holding them in the desired shape as they cool.

thicker pieces have harder times being adapted?

Hairdryer or really hot (not boiling) water bath for awhile should make the plastic pliable.

Slightly over-correct it when its in that state and let it cool. It should straighten out.

Just be careful when heating you dont go too hot.

When you cool them down again, make sure it’s REALLY cold. Like ice water or a freezer cold, not just cold. Otherwise, yeah, more heat.