Can a first void urine test miss and sti if you put a bit too much urine in and it dilutes it?

Maybe/not: The first morning void is usually more concentrated than later in the day. Modern tests for STIs rely on detecting germ DNA & are qualitative not quantitative. That means even a very tiny amount of the germs DNA will turn the test positive so dilution makes little difference. That said, any test can be produce inaccurate results & that frequency would depend on technique.