Hello,
I'm using an LPC54606 and I had the same problem.
I would like to use a Pint on PIO4_25 and I thought that it was possible bescause it's not totaly clear in User Manual.
Indeed when we look at the "Pin interrupt select registers" (table 262 pages 202 and 203), in the description it's not wrote that it is exclusively reserved for PIO0_0 to PIO1_31, espacially as 6 bits would have been enough to count from 0 to 63. And even more when it is writen in the datasheet (on page 67) that: "all GPIO pins can be selected to create an edge or level-sensitive GPIO interrupt request".
I don't know where I can talk about this but: I have also find a "misprint" in the User Manual (on page 313) about the reference table for clearing the timer reset. Indeed the table 234 is for CTIMER 3 and 4 not for CTIMER0 and 1. And the table 130 is for CTIMER0, 1 and 2 not for CTIMER3 and 4.
Best Regards,
Karim.