As others said, it is not a bug, also not an oversight since afaik you specifically have set the upgrade to require the player "being UKF", which means this behaviour is intended.
The question though is if the upgrade is part of the Cromwell's balance or part of UKF's overall balance. Apparently, the devs have decided that it is more important for UKF than for the Cromwell itself. I don't think this is the right call though, it should be available even if captured.
Seriously doubt that since game has a number of oversight that where not actually intended but where simply not noticed. I will continue to doubt that this behavior was a decision until someone can provide some proof that it was intended for the released product.
(I pretty sure most abilities are not created from scratch but edits of other abilities like "war speed" for instance or the reason behind it was so that the upgrade is not available to tank captured by UKF)
Stuka having 0 penetration for instance is a prime example.
I have no interest in arguing semantics on weather it qualifies as bug or not or even if it was decision at some point, it a abnormality that should be fixed and I am glad that you agree.
(edited)