I have had this happen all the time. I know what it is--and it occurs when you are typing formulas. As you type formulas, PA redetermines the validation for a variable. See my post:
https://powerusers.microsoft.com/t5/PowerApps-Ideas/Disable-check-for-errors/idi-p/45075
So when you are in the middle of typing:
UpdateContext({variable: tr...})PA will stall since it's now unsure if the variable takes boolean values or this broken text. Once you finish typing the formula, PA might be okay again. Other times, the yellow signs will remain. The fix I have found is to save the file and reopen it. As long as the variable is validated to take the same type of data in every instance, the app will open correctly without the yellow signs.
In addition to closing/reopening the app, it's a good idea to view the variables: where are they used and how are they defined. In one variable I had--I defined it with all the columns in one area and fewer columns in another one. It would work in 99% of situations, despite the different definitions. Then in editing a definition or adding another definition in another location, the yellow signs. It is just best to keep it all the same number of columns. To be exact, I consider a variable that is based on a record in a collection to be different than a variable that is based on an item from a gallery. The gallery item includes all of the controls with the data while the collection record is just data. Once I got everything redefined to be based on collections, I started having fewer of these situations.
The problem with UpdateContext() and Navigate(,,variable) is that the variables are scoped to one screen only, yet you need to define them in so many areas. A recent version of PA introduced the Set() function which makes a variable universally accessible on any screen.
As I was transitioning to change my old formulas that used UpdateContext to use Set() instead, I noticed that all instances of UpdateContext stopped working--which is okay. No error dialogues or yellow signs popped up saying they were incompatible--this is good for transitioning as you don't want to be slowed down. I recommend that you switch over to Set. View the variable wherever it is defined, including in Navigation formulas, and change it to Set instead.
Note: This is anecdotal, but I find that when writing a formula for Set(), PowerApps doesn't stall in the middle of typing "true."
Note2: I started editing certain variables outside of PowerApps and copy+pasting it back, but it defeats the purpose of using the formula bar in PA: you miss out on the color coding, spacing, referencing, and autocomplete. However, in this method, I can't break variables. https://powerusers.microsoft.com/t5/PowerApps-Ideas/Disable-check-for-errors/idi-p/45075
