Are you building these tool-chain generations or using pre-built ones?
In either case - -
Is there a chance that glibc is never getting re-downloaded and re-built?
Since everything is dependent on the tool-chain - a "make clean" on the entire Buildroot system is required when making a change to the tool-chain.
Even when just selecting a different external tool-chain.
I am sorry that I can't be running these Buildroot builds in parallel with you - but I am still trying to solve some local problems in getting a new RAM kernel built.