i must add this:

after a fresh install of opensuse nothing is set in advanced settings. everythings is empty.

this was not the case with kde 4 !

(In reply to Philippe ROUBACH from comment #0)
> STEPS TO REPRODUCE
> 1. go to settings > composer > autocorrection
> 1. set "set replacement and exception for language" to "french"
> 2. button "Apply" stays grey
> 3. quit kmail
> 4. launch kmail
> 4. go to settings > composer > autocorrection
> 
> OBSERVED RESULT
> 
> all about autocorrection settings disappeared
> 
> EXPECTED RESULT
> 
> all about autocorrection settings is recorded
> 
> SOFTWARE/OS VERSIONS
> Windows: 
> MacOS: 
> Linux/KDE Plasma: 
> (available in About System)
> KDE Plasma Version: plasma 5.15.2
> KDE Frameworks Version: 5.55.0
> Qt Version: 5.12.1
> 
> ADDITIONAL INFORMATION

We can define which word will be replace by other word, you can change language and you will see specific list.
=> for sure when we switch to french we will not activate apply button.
But if you change list of word it will change.

In identity you can specify which language using.
so it's not a bug here.

you miss completely the bug !

i did not say you can't set advanced settings. i said nothing is recorded. at next launch of kmail you see all your advanced settings disappeared.

(In reply to Philippe ROUBACH from comment #3)
> you miss completely the bug !
> 
> i did not say you can't set advanced settings. i said nothing is recorded.
> at next launch of kmail you see all your advanced settings disappeared.

nope it works here.
All checkbox is checked as before I close kmail
I added a find/replace word and I can see it after reload.
So no all works here.

i get this problem since i installed opensuse 15.0 in 2018 august with an update from 42.3.

not any kde update(using opensue argon repos) since this day have fixed the problem.

perhaps a corrupted config file but what file or part of file ?