First, I can confirm that UART transmit the problem is real. We have fixed it and it will be available in MPLAB X 5.50. Possibly sooner if the MPLAB X team has already implemented the ability to download tool data packs (similar to device packs). I'm still investigating that.
The simulator team is a bit busy at the moment, and this issue had a few red-herrings that we wasted time chasing after, so it took us a few days to be able to say anything definitive here.
In post #3 Ric asked a very pertinent question. And the answer is: No, the simulator team was unaware of this problem. The link given in post #2 was not in the simulator forum so we didn't see it. And it would appear the OP must not have created a support ticket. Or if he did it didn't get to us.
This issue is almost identical to another issue that arose for PIC16 about a year ago that we fixed. So it is entirely possible that someone tried to tell us about this PIC18xxk42 issue but we missed it because... "We already fixed that issue."
With respect to post #8. Early on in the simulator effort we recognized that darn near every device had a "unique" oscillator scheme. The estimated effort for implementing all these oscillators was daunting. So we opted to just have the user tell us what their resultant Fosc ends up being. Modelling the various oscillators is still on our "infinitely long list of simulator improvements". And, of course, packages like MCC are completely unaware that you are using the simulator so they end up further masking the "simulator needs you to tell it Fosc" issue by making you think you've already done everything you need to set up the oscillators.