There certainly was evidence that demonstrated no change in drop rate at the end of a farm a few years back, compiled by people who are still known in the SWGOH community. Try this link, for instance.
There was a wider project on a now defunct Discord server (Planet Coruscant, I think, but I don't remember for certain) that I contributed to that measured the same data and had a sizeable body of evidence that showed the drop rate was not affected by where about in the farming journey a player was.
The problem with "proving" or "disproving" things like this is that anyone who believes the drop rates ARE adjusted trying to convince someone who doesn't believe that (and vice versa) is always swimming against the tide.
If someone is convinced that mean, horrible CG are out to get them and do manipulate the drop rates, no amount of data will ever convince them otherwise. Conclusions based on huge amounts of data will be dismissed as "RNG" or "not enough to make a conclusion", either through a lack of understanding of how statistics work and/or deep-seated conscious and unconscious bias against CG.
As a mathematician, I'm in the other camp - of course. And I've never yet seen any credible claim on the forums or elsewhere to convince me that these ideas are anything more than a bunch of conspiracy theories. People make statements like "I've been getting mostly 1 shard per run, hardly ever 2" or "my luck with slicing mods is horrible" and expect them to be given the same weight as genuine pieces of tracked data, and they NEVER come back with any actual hard data when challenged to do so.
Taking a non-accelerated ship or character from 95/100 to 100/100 should, on average, take 15 attempts. Some will be completed faster, some will be slower. Some people will remember the slower farms bitterly and will conveniently forget all the farms where it took less time than expected.