Thanks. Hopefully fixed for the next release.
The main issue was that the (hidden) status effect that tracks Ovilium was prematurely expiring over time instead of sticking around throughout the pregnancy (like it is intended to). This might have been due to a side effect of setting the status effect's timer to fix a different bug of the effect never expiring (so an over-correction of sorts). Now, with some testing, the effect should only expire, be it automatically or via a timer, only if the character no longer has the Ovilium egg pregnancy (basically after laying the eggs). With the status effect sticking around during the pregnancy, the large egg event should now correctly trigger when the character is inseminated while pregnant with Ovilium eggs.