Jumping back and forth between firmware versions isn't helping anyone narrow down anything. Unless you're actually fully resetting to defaults?
Are you using AX mode with an AX capable client in the 2.4GHz Band in your testing?
I'm using AX mode (not that it matters, see below) with an AC client in mine.
Impossible to have 300Mbps connection speeds with a single antennae, from my understanding.
https://www.duckware.com/tech/wifi-in-the-us.html
Again, I'm not doubting what you are seeing, but you are just not giving enough quality information right now.
And, importantly, on my router, the performance is as expected and/or better, in the 'as designed' usage mode.
@sturmstar, I would never be willing to go back to an older version. Particularly not to 'just' test, when the current firmware is obviously superior in my usage/network environment. Sorry.
Note that my RT-AX88U HWvA1.1 (China) router first breathed life with 384.14 Beta 3 with a full M&M Config, and what I am getting out of it now is even better.