It seems the landscape painting and sculpt tools have been broken in this version. I was previously using 4.16 and I didnt notice this but in 4.18 pressing shift and click does work but only at first. If you leave it held it will not recognise it as held and result in normal sculpt or add paint operation. Undo also doesnt work correctly . on multiple undos it removes a lot more than it should. it broke the landscape on the component line so that they were separated and that results in sculpts not adding correctly, a steep near 90 degree angle is created.
I dont think its my installation but it might well be.
I was not able to reproduce this issue on my end. Are you able to reproduce it in a new, blank project? Additionally, are there any other settings that may be affecting the landscape?