I’m having a go at “hard” ginger beer. My ginger bug was nice and fizzy - but before I added it to the demijohn I mixed it with the lemon juice. Was that a really bad idea? Not much sign of fermentation so far....
The lemon juice should add anti-oxidant properties, vitamin C and of course a complementary flavour, it shouldn't affect fermentation. I've been adding lemons to my ginger beer for years.