Based upon my motorcycle experience, the defective needle would sometimes seal and sometimes not. I have never owned a four wheeled vehicle with carbs. My 1971 142 had D jet so its been einspritzen since day 1. But, I have had numerous friends with SU equipped vehicles. Non sealing needle jets was a rather common theme on older SUs and in the '70s, the Grose-jet was the preferred solution to needle jet problems - amongst my friends. I see they still make them:
https://mossmotors.com/386-390-grose...or-needle-seat
I have no hands on experience with the Grose-jet.
If you have a paper element style fuel filter it is unlikely that dirt is causing the problem. Mesh fuel filters are less effective. As suggested by Burdekin, too much fuel pressure can be a problem. The front needle valve may be the weaker link and overflows first to relieve excessive pressure. Check with a fuel pressure test gauge. The running pressure should be less than 3.5 psi on SUs.