GZK 1 mm Orange - has anyone had it stretching its active length?

I cut a 10/7.5 mm taper bandset, active length 210 mm. It was shooting OK with 8-12 mm steel with 4.25x draw and 9.7 lb draw weight. e.g. 9 mm steel at 219 fps. About 40 shots in all

So then I tried with 4.5x draw and 10.3 lb draw weight and the speeds were up, e.g. 9 mm steel at 230 fps, but after about 20 shots I noticed that the bands had stretched their active length from 210 mm to 214 mm and 218 mm respectively.

This also happened in exactly the same way with a second bandset with 9/6.5 mm taper.

I had never tried this GZK 1 mm Orange rubber before and the bandsets were cut from the start of a new roll from Everything Sling (UK).

I guess I'd better try some rubber from further into the roll.

Can anyone shed any light on this, please?