I suddenly remember that once I had a RO unit that was not properly mounted.
There is what they call an ASO valve, that was in fact mixing the RO water with the input one. Something that looks like that:
https://www.grandado.com/products/4...eu5ozXJh1QntSIDVftHiT0MdrzG3lkoxoCnrMQAvD_BwE
It can be standalone, or with an electric power on/off, a kind of tiny black block. I had to discard it, and repipe a bit the RO membrane then it worked smoothly.
Basically you want to have a linear installation, from the tap to filter 1, then filter 2, then filter 3 eventually, then the pipe enters the RO membrane from the bottom, and you have 1 exhaust pipe and 1 RO pipe. On some models you have to remove the 'recycling' option, that I saw in Asia. It basically tries to re-enter a percentage the waste water of the RO into the input water going through the RO.
When you have photos it will be easier, but check as well if the installation is 'linear' like I say, or if there are some strange ASO/blocks around the way, or 'return pipes' from a later stage on the line to an earlier on