I had the same issue and did the same things and it always came back. I changed my RODI filters (even though reading o on the TDS) and haven't seen Dino's since.
I have not... I was going to buy new filter for my BRS rodi. Start there and order a UV. I did buy the RODI 2nd hand so... wonder if that’s the issue. 0 TDS but like stated above he changed filters and it went Away