I've had the same experience when my ISP put me behind a CGNAT: the port table got reset every other day, and some other customer could have taken the port you

I've had the same experience when my ISP put me behind a CGNAT: the port table got reset every other day, and some other customer could have taken the port you wanted at that point, applications with ports hardcoded over number 2000 did not work because they could not be requested, people with PPTP VPNs could not use them anymore as GRE didn't work. Plus, enjoy getting banned from things because you share your IPv4 with plenty of strangers. The solution from the ISP was to give those who complained a "fixed IPv4 plan" for free, since that pulled you out of their IPv6+CGNAT deployment plan, at least for the moment.