SPIN assert not triggered

I am trying to understand why the assert in this model isn't triggered.

ltl { !A@wa U B@sb && !B@wb U A@sa }

byte p = 0
byte q = 0
int  x = 0

inline signal(sem) { sem++ }
inline wait  (sem) { atomic { sem > 0 ; sem-- } }

proctype A() {
    x = 10*x + 1
    signal(p)
sa: wait(q)
wa: x = 10*x + 2
}

proctype B() {
    x = 10*x + 3
    signal(q)
sb: wait(p)
wb: x = 10*x + 4
}

init {
    atomic { run A(); run B() }
    _nr_pr == 1
    assert(x != 1324) 
}

Clearly, there is an order of operations that produces the final value x = 1324:

Initially x = 0
A sets x = 10*0 + 1 = 1
B sets x = 10*1 + 3 = 13
A and B allow each other to proceed
A sets x = 10*13 + 2 = 132
B sets x = 10*132 + 4 = 1324

Solution

The assertion isn't triggered because it is "never reached" when the solver proves that the property

ltl { !A@wa U B@sb && !B@wb U A@sa }

is true.

Take a look at the output that is given by the solver, it clearly states that:

it is checking any assertion, but only if within the scope of the claim:

Full statespace search for:
    never claim             + (ltl_0)
    assertion violations    + (if within scope of claim)

the assertion isn't reached:

unreached in init
    t.pml:27, state 5, "assert((x!=1324))"
    t.pml:28, state 6, "-end-"
    (2 of 6 states)

You can use the option -noclaim so to check the model only for the assertion, which is then easily proven false:

~$ spin -search -noclaim t.pml 
ltl ltl_0: ((! ((A@wa))) U ((B@sb))) && ((! ((B@wb))) U ((A@sa)))
pan:1: assertion violated (x!=1324) (at depth 13)
pan: wrote t.pml.trail

(Spin Version 6.4.8 -- 2 March 2018)
Warning: Search not completed
    + Partial Order Reduction

Full statespace search for:
    never claim             - (not selected)
    assertion violations    +
    cycle checks        - (disabled by -DSAFETY)
    invalid end states  +

State-vector 36 byte, depth reached 15, errors: 1
       48 states, stored
        6 states, matched
       54 transitions (= stored+matched)
        1 atomic steps
hash conflicts:         0 (resolved)

Stats on memory usage (in Megabytes):
    0.003   equivalent memory usage for states (stored*(State-vector + overhead))
    0.286   actual memory usage for states
  128.000   memory used for hash table (-w24)
    0.534   memory used for DFS stack (-m10000)
  128.730   total actual memory usage



pan: elapsed time 0 seconds