Ran into this issue and resolved by re-opening the project. A save dialog appears to save material changes, though Save All was just performed. Everything good after revert to saved. Seems as though some materials need a spanking.
Issue still occurs in 5.5.4.
On 5.5.4 as well. This appears to have worked immediately for me as well. Doing it in the project had no effect but this appears to have had an effect.
Just deleted this forlder. Itโll probably be under your Users/[username]. Protip you can type in %APPDATA% in file explorer on Windows to get to your APPDATA location quickly.
\AppData\Local\UnrealEngine\Common\DerivedDataCache
UPDATE: This only worked for some of us on the team. Maybe its tied to multiple steps that arenโt clear.