I don’t think the heat bed is failing as you can print cold with PLA, I would check for either rare movement, obstruction at the rear(have you fitted a temp/humidity gauge at the rear that may be rubbing slightly) or maybe a loose pulley that would be more noticeable when the print head was closer at the back than at the front, even something like damage or something stuck on the belt that only runs over the pulley when the print head is at the rear of the machine, the filament waste can be flung all over at times It’s a very strange problem but looks to me that something is happening when printing towards the rear and the non smooth running of a belt over the pulley could cause a jaggedly finish say if a strand was stuck to the belt and running over the teeth of the pulley.
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