Scala plugin does not seem to be able to resolve symbol "++:" for some reason

IntelliJ IDEA:

IntelliJ IDEA 2019.2.3 (Ultimate Edition)
Build #IU-192.6817.14, built on September 24, 2019

Scala plugin:

2019.2.28

Scala version:

2.12.7

After the recent IntelliJ update, I have been encountering "Cannot resolve symbol X" problem.

For instance, when I try to use ++: of "scala.collection.immutable.Map", the IDE complains about the method,  but still compiles with no errors.

How can I fix this?