Remember that play pens are things we humans place to restrict a rabbit’s access. For a rabbit, a play pen is this weird thing that allows them to see the other side but for some frustrating reason they aren’t able to get to the other side from where they are (e.g., all the instances of rabbits chewing their play pen even though it’s open). Typically, boredom, frustration, and/or visually seeing the outside will facilitate pen chewing.
There are some suggestions here to help dissuade it: https://wabbitwiki.com/wiki/Training_FAQ#My_rabbit_keeps_me_up_by_consistently_chewing_his_cage_bars._How_do_I_make_him_stop?
Also, I notice you mention an enclosure and a play pen, and it sounds like you let your rabbit out of the enclosure into the play pen? Generally, a play pen is the minimum sized space you’d want a rabbit in for any amount of time, so if his enclosure is smaller than a playpen and you’re keeping him in there overnight or when you cannot supervise, please consider just transitioning him to the playpen 24/7 to ensure ample exercise and space in general.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.