What happens if you swap the bulbs from left to right to see if the fault follows?.
I've had partially faulty bulbs do funky things :/
I also had to replace my front and rear indicators a while ago for some cheapies off eBay as that were so old the plastic was decaying and cracked and the metal bulb holders wouldn't work correctly no matter what I did.
I am a fan of LED bulbs but couldn't be arsed to mess around so standard
https://shorturl.at/c0XxS