For a given agent, I have 1 custom capability defined:

variable_capability = 67890

For a given plan, I have 2 plan variables defined:

variable_plan = 12345 ${bamboo.capability.variable_capability} ${bamboo.other_variable_plan}

other_variable_plan = ABCDE

In that plan I have a single stage running a single job with only one script task executing:

echo %*

with the following arguments:

${bamboo.other_variable_plan} ${bamboo.capability.variable_capability} ${bamboo.variable_plan}

The result of execution I had with Bamboo 4.4.5:

ABCDE 67890 12345 67890 ABCDE

The result of execution I now have with Bamboo 5.1.1:

ABCDE 67890 12345 ${bamboo.capability.variable_capability} ${bamboo.other_variable_plan}

Unless recursive variable resolution was a side effect in 4.4.5 I feel that this a bug in the new version.