I published this over in Beta and realized it more properly belongs here. Apologies for the double post:
Running main version 6.0.2 build 4.10.2019 here. I am having the same problem. I use Notes plus primarily by creating text boxes and typing my notes about my activities in there. For about two weeks now I have notices that when I create text, close the active text box, then leave the app and return one or more text boxes are no longer visible. Tapping the area where the missing text was located causes the little check in a circle icon to appear at the place where the upper right corner would have been, but it disappears when tapped. No text box outline, box menu, or text appears. This is a huge problem - I have based my field work on Notes Plus almost since it first came out. I can no longer rely on it and have returned to paper notebooks until this is resolved. If it is not resolved quickly I will have to abandon the program altogether. Help!