It's not being warned of routinely because it's not clear yet that it actually is even linked to the vaccine far less being listed as an official side effect.
Nor is it clear what the risk factors are so as to warn anyone specifically. There is nowhere near enough data, certainly in the public domain, to even confirm any age or sex link. It is all speculation at this point.
The only condition that would be a known contraindication would be a previous history of heparin induced thrombocytopenia which is also very rare. I am sure you would be well aware if you'd had that and you would be advised not to have it if you had that particular condition.
It's not to say that it won't eventually be confirmed as a side effect but equally it might be disproved. The apparent association could very easily be due to chance, sampling bias or confounding.
I am happy to await the U.K. and EU regulators to do their job and examine the data. I do not think it's naive to trust agencies that are specifically tasked to do this job.