Whilst I've normally seen it with addressable systems and suitable C&E or mix of addressable with conventional spurs (e.g. C-tec hush button and their current Hush-Pro) it's not unheard of to have a single system rather than two mixed.
The mixed system spec is only a suggested minimum for simultaneous evacuation and not a rigid specification. Depending on the risks covered there may be need for coverage of a common system beyond a flat hallway, such as external fire spread issues, heritage age buildings where there are voids and poor ceilings that could spread fire before a hallway heat would trigger, etc.
The mixed system concept is to provide early sitewide warning before compartment of origin breaches without undue false alarm risk leading to lack of confidence in the alarm and possible interference with it, the local system for occupier of origin flat life safety only disrupting the one flat if they burn the toast/leave the bathroom door open etc....
The set up you describe would satisfy communal early warning and local early warning in one as you have common detection before breakout from the flat AND local smoke detection and warning in internal escape routes (& more)for the early warning in the flat of origin so no real need to change it or add other bits from that point of view. It's only vulnerability is the false alarm risk and knock on effects from that which as it's not causing any issues isn't a major issue in the short term. Long term once the conventional kit was end of life I'd consider upgrading to addressable to be able to program out the worst of the false alarm risks but that's about it.
Is it providing a lesser level of protection than that in the 'usual' set up, is it not in line with the appropriate evac strategy and is that difference in protection putting relevant persons in danger of serious injury or death? If you are saying no to these questions then it's not 'wrong' just a different way of achieving the right precautions. I'd still advise that this rationale for accepting it is robustly explained in documentation relating to the fire safety arrangements though due to the various changes in legislation putting you very much on the back foot if you deviate from guidance.
