The manual states of 03 (interrupt register)

'After the register is read the corresponding bit in the interrupt request register is reset'

However the code as implemented only seems to update this on a 1 ms timer.

In addition the uart0_int value when computed considers the int_mask value at the point of computation, but does not appear to recompute the mask when the int mask changes only at the next ms timer.

It looks to me like the register state computation (as opposed to the timer expiries) is happening in entirely the wrong time domain, so that code which polls the register until it hits FF handling events spins for a millisecond incorrectly.