in the Niagara System & Emitter windows the compile button will be missing if small toolbar icons is enabled. The icons work correctly if this is not enabled.
Found in 4.19 CL# 4033788, 4.20p1 CL# 4112139, 4.21 CL# 4115885
The compile button will be missing from the tool bar
all buttons on the tool bar will display their small version
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-60115 in the post.