It is not entirely clear what your question is. RCBOs that only break the phase cores are common, certainly on 3 phase, but they are not a wonderful idea in systems with upstream RCD protection, delay or otherwise, as an NE fault (or an LE fault and a resisting LN load) is not disconnected by the operation of the RCBO, and the upstream protection fires as well.

Avoiding this sort of problem  is part of good system design. 

If you are really asking why do the regs not prohibit it, the answer is that there may be situations (not yours presumably) where the operation of the upstream RCD is desired for certain types of fault and there is no good reason for the rules to be overly restrictive - the regs are not a recipe book after all . It is a matter for the designer when capturing the system requirements and selecting the components to weigh up such factors as how much of a problem is nuisance tripping, versus the danger of a part-live part dead installation..

Some folk seem to struggle with protection on systems with TT earthing, but a tiered approach to both RCD trip thresholds and delay times, and yes L and N breaking at every tier, are the correct way forward.
Mike

Basically I’m coming across installs on tt systems where the time delayed RCCB is down stream and consumer unit has rcbos single pole installed. The faults are bypassing the rcbos and taking out the time delay RCCB. Where as if you have double pole rcbos installed it disconnects them under fault conditions. Only on tt systems only.

Basically I’m coming across installs on tt systems where the time delayed RCCB is down stream and consumer unit has rcbos single pole installed. The faults are bypassing the rcbos and taking out the time delay RCCB. Where as if you have double pole rcbos installed it disconnects them under fault conditions. Only on tt systems only.

That is entirely possible, and  arguably phase only breaking RCBOs are not a great design choice for something like a dwelling , where normally you'd prefer to keep the lights on if the power goes off for example.

As above it is due to the fault pushing current in the neutral earth loop which is not interrupted. . Once the fault is cleared, do you measure the N-E offset voltage in these installations, or confirm that there is no significant standing current into the earth electrodes under normal operation ?

Large NE offset voltages can be an early warning of distribution side earth problems.

Mike.