Yeah... you can't just pay
any higher amount... there is a certain minimum threshold that you have to cross to get it accepted.
Unfortunately, it's not a "fixed" amount as such... it depends on transaction sizes and fee rate used for the original transaction etc. So, trying to do this manually can require a bit of calculating (and/or experimenting) to make it work.
I believe it is done this way to prevent a possible DoS attack scenario, by making it "expensive".
I did a manual RBF about 2 weeks ago during the previous price pump... and do know that I was able to pay a lot less than the "minimum" that Bitcoin Core was trying to use... from memory it was still 5-6x the original fee... ~400 sats vs. ~2500 sats... for a transaction that was around 400
vbytes (~1600 WU)...
Bitcoin Core wanted me to pay like ~50000 sats as the "bump fee"
But I suspect it was also doing confirmation time calcs and trying to get within 25 blocks etc.