Should I say anything having found out (accidentally) problems with my teaching?
Consider that it may not have been the quality of your teaching that was the issue.

From your description it sounds as if the course was a shambles (probably best not tell us what university this was). Asking an inexperienced teacher to create 32 hours of Masters-level lectures, from scratch, to a large-ish class, without (from your account) any significant discussion of curriculum or assessment?! And hourly paid?  There are circumstances in which each of these would be semi-reasonable, but your account paints a very disappointing picture.

It's quite possible, for example, that the ‘obvious reasons’ are that the course leader/convenor/chair discovered after the fact that you'd omitted to cover a topic that was deemed vital for that course – perhaps the curriculum is constrained by pre-existing assessment or other course dependencies – but which the course leader had never told you about.

The above is of course speculation, but it might be useful for your peace of mind if you can discover if the issue was the quality of your teaching or some other organisational issue. If you are sensitive enough to your teaching, and to its reception, to be able to detect that it improved notably between the first and second cycles, then I would guess that it was at least OK.

Bear in mind, however, that if the real problem was that the course was badly organised, and you badly briefed, then the course leader and their colleagues might be rather sensitive or defensive about it, and you might have to bear that in mind when discussing it.  Also, bear in mind the possibility that – unfortunately and very discreditably – the course administration may have dealt with the problem by blaming the conveniently temporary and now-departed teacher: ‘an inexperienced teacher...; I'm sure I told them what to cover...; anyway, they never asked me....’

If anything like that is true, then it seems unlikely to me that there'd be any longer-term comeback to you. It's even possible that they think of you as someone who was placed in an impossible position but still managed to achieve something.

If you only want to find out details, rather than complain, then an informal setting for a chat might be more informative than a multi-person meeting; and if you know any permanent staff who taught on the course, they might be more usefully indiscreet than the course leader.