Is there a bug when we assign new value to $fU pseudo-variable multi time?

My scenery is according below:

In a point on config file, let us that $fU value is: 7777. When I do assign:

.....

$fU = "5555" + $fU;        ### Here $fU = 55557777

.....

In another point, let us that value $sht(htble=>$var(nnnn)) hold is: 44444444. And I do:

.....

$fU = $sht(htble=>$var(nnnn));     #### Here, it seems that this is happening $fU = 5555777744444444

.....

When I turn on ngrep analysis, I catch From header:

.................

.................

From: "User Name" <sip:5555777744444444@IP-Address>

.................

.................

This two assignments are realizing on same route block.

So, it seems to be a bug.

Best Regards,

CMA