For some reason, it appears that manually setting the focus (whether it be user, keyboard, or both) on an editable text box in UMG causes the caret to blink slowly. However if you were to manually select the text box with your mouse first, then manually setting focus at any point in the future will cause the caret to blink at a normal speed.
Versions tested (issue occurs in all versions):
4.19.2 - CL 4033788
4.20.3 - CL 4369336
Result: Notice that the editable text box's caret blinks very slowly.
Expected result: The caret should blink at the usual speed.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-65468 in the post.