I'd be genuinely interested to see where that's written, propnut. There's nothing about it in the UK CAA Civil Aircraft Inspection Procedures. What they do state, is when tyres are being leak-checked after mounting on the wheels, the cap is
not to be installed to allow the valve core serviceability to be proved. No mention of ever routinely checking the yellow 'primary' installed caps with leak detector spray. It would obviously be a waste of time if the valve core is working properly so how could it tested for serviceability? I've never seen caps being specifically called up for any maintenance other than general visual inspection. Considering some tyres are inflated to 200 psi it's hard to follow that a 'primary' seal wouldn't warrant any kind of function check ever.
This subject came up on Pprune about ten years ago and it turned into quite an entertaining thread.
See here:
Valve cap missing