The NEO Flex portafilter and cylinder are separating.

Note that unless you are seeing a clear water leak between the cylinder and portafilter, the o-ring is doing its job and does not require replacement. Instead, this separation is more likely related to your actual brew process.
Due to the minimalistic design of standard Flairs, there is no locking mechanism between the portafilter and cylinder. As such, they will separate/pull-apart if given the opportunity. During brewing, the act of applying that downward force will hold them together, but afterwards (or during, if you've suddenly let up on the lever), any pressure remaining in the brew head will force them apart in an effort to escape.
At the end of your pull, just make sure you are holding down the lever until pressure has naturally dissipated and are expelling all brew water, then slowly raise the lever to terminate and avoid that rebound affect. Even if they do slightly separate, there is a designed, extended lip to prevent full separation.
On the NEO, note that a too fine grind size or the presence of a clog may be exacerbating this attempted separation, as the pressure being built within the brew head will be very high.