Test failure on `ubuntu-26.04`

[cqc-alec]

One of the C++ tests consistently fails when run on the ubuntu-26.04 runner.

See e.g. https://github.com/Quantinuum/tket/actions/runs/28235360955/job/83660380480?pr=2192

-------------------------------------------------------------------------------
Scenario: Simple MultiplexedTensoredU2Box decomposition with rotating string
          and 4 controls, 4 targets, using all strings
-------------------------------------------------------------------------------
../../../test/src/Circuit/test_Multiplexor.cpp:550
...............................................................................

../../../test/src/Circuit/test_Multiplexor.cpp:621: FAILED:
  REQUIRE( check_multiplexor(op_map, *c) )
with expansion:
  false