When the site is on a small slope, sometimes even invisible to the eye, when laying out a pond, do not forget that groundwater does its job in the autumn-spring period, and such surprises occur.
The photo clearly shows how the film floats up from excess pressure. Usually drainage and a pump help out, you can solve it constructively. But it is better to initially carefully make the right choice of location for your pond.
When creating complex objects, you should not skimp on research, because design allows you to take a lot into account.
The photo shows how water lifts the waterproofing, and a small slope covered with forest drains the surface layer perfectly. In pictures 1-2 you can see the problem, in the third - its solution.