Hmm...

Generally I agree, once the stuff you add gets up to temp you should be sitting at the same temp.

But...

Do you use a mister or anything that adds moisture to the viv? Maybe the increased surface area is allowing for more evaporation which is bringing the temp down?

Plastic doesn't hold onto heat well, if you are using bulbs/CHEs (ceramic heating element) to provide heat from above maybe its bouncing off the plastic rather than sinking into the substrate?

You say "the hot end" which leads me to believe you are using a pair of bulbs/CHEs to make a temp gradient in the tank. Perhaps by using top heat to produce an ambient temp in the entire tank and a UTH(under tank heater) to provide a hotspot in the hide you can mitigate this issue. In general I would recommend this approach rather than relying solely on top heat. You can end up really cooking the top of your hide trying to get the inside to 90.

By adding earthenware decor, rocks, hides, water dishes, etc. you could maybe impart more heat retention into your tank to offset the apparent issue the plastic is giving you... I would say swap the plastic out for real plants but they wouldnt last one night in a tank with a BP.

As a side note your problem, I think, points to the fact you are not using a thermostat? I would HIGHLY recommend using a thermostat to regulate your hotspot temp as fluctuations here can quickly stress your snake.

hope this helps, keep us posted, oddities like this are really interesting