Pyproject.toml is being used by increasingly many, often not build related tools. Each of the tools reads the file using a parser of their choice, implementing an arbitrary TOML spec version. This creates ambiguity when reading pyproject.toml: some tools interpret it as TOML v0.5.0, some as v1.0.0, some will raise an error parsing heterogeneous arrays, some will not, etc.
This is unfortunately already the case: the two most widely used libraries (toml and tomlkit if I’m not wrong) are at TOML versions 0.5.0 and 1.0.0-rc.1. A valid package built with Poetry may cause pip to throw an error due to TOML syntax not valid in v0.5.0. The problem will only get worse as the TOML spec advances and gets new versions.
I think it would be a good idea for PEP 518 to state that pyproject.toml must be parsed, for instance, strictly as TOML v1.0.0.